I was reading the specs of a few zoom eyepieces, and in every eyepiece, there is a term called 'apparent field of view'. I looked up and searched about it and figured out that if you divide this by the magnification caused by the eyepiece, you get the real field of view which is basically the diameter of the circle visible in the telescope. But one thing that I was unable to find that was what would be a good field of view for viewing planets through a zoom eyepiece, so this is my question.
Planets are very small objects, so they'll fit into whatever AFoV the eyepiece provides.
One highly-rated planetary eyepiece, the Vixen HR, has an AFoV of only 42°.
The main problem with low fields of view and high magnifications, however, is keeping the planet in the centre of the field of view.
The hardest type of mount to do this with is a manual altitude/azimuth mount, because you need to move in two axes at the same time.
In your case, with a manual German equatorial mount, you only need to worry about tracking with the right ascension (RA) axis, and your mount may well have a slow-motion control to help you out with that.
So, when looking to buy a zoom eyepiece for looking at planets, don't worry about the FoV it has. Almost all zoom eyepieces have quite small fields of view when compared with wide-field fixed-length eyepieces. I believe there's only one zoom - a Leica - which has a comparable FoV.
(And if tracking planets at high magnification manually still isn't for you, then consider a wide-angle eyepiece (82° or greater) to give yourself more "drift time" before you have to nudge the telescope)