You should take the inner and outer edges of the habitable zone of the Sun as your model. You say your star has luminosity of 20. If that is 20 times the luminosity of the Sun, there is a comparatively simple method to adjust the size of the Sun's habitable zone to fit your star.
The square root of 20 is 4.47213. So you multiply the inner and outer edges of the Sun's habitable zone by 4.47213 to get the inner and outer edges of your fictional star's habitable zone.
So what are the inner and outer edges of the Sun's habitable zone?
Here is a link to a list of about a dozen scientific estimates of the inner and outer edges, or both, of the Sun's habitable zone made in the last 60 years.
And some of the estimates are very different from some others.
So which estimates do you use in your story? The ones which are best for your story if you are a typical lazy science fiction writer, or the ones which make the most scientific sense to your if you are a conscientious science fiction writer.
And to find the estimates of the Sun's habitable zone which seem most scientifically plausible to you it is necessary to read the scientific articles proposing each estimate.
If you are certain you want one and only one habitable world in your fictional star system, you cans simply put that world at what I call the Earth Equivalent Distance or EED of the star. The EED of a star is the distance at which it receives the exact same amount of radiation from its star as Earth gets orbiting at a distance of one Astronomical Unit or AU from the Sun.
Divide the star's luminosity by that of the Sun, and find the square root of that ratio, and then multiply 1 AU by that square root of the ratio, and you will get the EED of that star.
A star with 20 times the luminosity of the Sun would have an EED at 4.4721359 AU.
A star with 20 times the luminosity of the Sun would be in between an A3V star with 16.98 times the luminosity of the Sun and an A2V star with 22.99 times the luminosity of the Sun.
For context, the planet is a water planet (thus the Albedo of 0.08), with a relatively Earth-like atmosphere.
I wonder what you mean by a "relatively Earth-like atmosphere."
Do you mean an atmosphere with a high oxygen content, so that humans can sail on the surface without wearing breathing gear, and so there can be multicellular plants and animals native to the planet?
If so, you should know that Earth has had a oxygen rich atmosphere for a "mere" 600 million years. It took four billion years before that for life to start and then for photosynthetic lifeforms to evolve and start producing free oxygen and for a large amount of oxygen to eventually build up in the atmosphere.
So a planet with an oxygen rich breathable atmosphere should be billions of years old, and its star should have been shining with a fairly steady luminosity for those billions of years. That means the star should have been in the main sequence phase of its existence for all those billions of years and has not yet become a red giant star, destroying all life on its planets.
As it happens, the amount of time that a star spends on the main sequence depends on its initial mass. The higher the mass the shorter the time spent on the main sequence.
Stephen H. Dole, in Habitable Planets For Man (1964), a very useful book for science fiction writers, discussed the types of stars suitable for having planets habitable for humans.
On pages 67 to 72 he discussed the s type of stars suitable for having habitable planets.
Dole decided that a planet might possibly develop a breathable oxygen rich atmosphere in only 3 billion years, 0.75 as long as it took Earth. And on page 68 he says that only main sequence stars of spectral class F2 and stars of lower mass can remain the main sequence for 3 billion or more years and so possibly have planets with oxygen rich atmospheres.
Spectral class A stars are more massive than spectral class F stars and so remain on the main sequence for short periods of time.
Vega (Alpha Lyrae) is a class A0V star with 40 times the luminosity of the Sun.
At present, the star is about 455 million years old. Vega will leave the main sequence in about 500 million years and bloat into a red giant before expelling its atmosphere and evolving into a white dwarf encircled by a planetary nebula.
Fomalhaut A is an A3V class star with 16.63 times the luminosity of the Sun.
Fomalhaut is a young star, for many years thought to be only 100 to 300 million years old, with a potential lifespan of a billion years. A 2012 study gave a slightly higher age of 440±40 million years.
A class A star with 20 times the luminosity of the Sun would be between Vega and Fomalhaut in mass and lifespan, and so would not remain on the main sequence for more than one billion years.
A writer who doesn't care about how low a score his story has in the scale of science fiction hardness
will go ahead and put his planet in orbit around an spectral class A star without a thought.
A writer who wants a higher hardness score will have to imagine that a habitable planet with an oxygen rich atmosphere orbiting a class A star must have been terraformed by an advanced society in the past.
I note that as early as the 1950s Robert A. Heinlein mentioned that spectral class G stars were the best for having habitable planets in his juvenile novels Starman Jones (1953) and Time For the Stars (1956). While on the other hand Timothy Zahn's "Music Hath Charms", Analog, April, 1985 mentioned habitable planets orbiting Vega and Algol, both quite unsuitable.
I also note that a habitable planet orbiting a spectral class A star needs to have a much greater ozone layer than Earth does, to protect the surface of the planet from the greater amount of ultra violent ultraviolet light it will get from its hotter star.
And the Worldbuilding Stack Exchange is a good place to ask questions about creating fictional worlds and societies.