I would simplify it to you have advantage on rolls when casting on the deities' plane, no effect if you are on a plane that has one or more of the descriptors of the deities' plane (lawful, chaos, good, evil, neutral), and disadvantage if you are on an outer plane with none of the descriptors of the deities' plane.
I unequivocally recommend throwing out that bit about "clerics and magic distant from their home-plane suffer drawbacks." Terrible design for a setting all about exploring the planes. Never played with it in AD&D, didn't play with in in 5e.
OTOH, what I *do* recommend paying attention to are spell changes on the planes, spell keys, and the cleric version "power keys" (in my version spell keys & power keys are just different terminology to describe the same thing). Rather than going strictly by what the boxed sets say, I recommend using the boxed sets (Planes of Law, Planes of Chaos, Planes of Conflict) as inspiration for your own adventures & specific planar sites.
I'd also considered equating spell and power keys, which helps keep the mechanics the same, since divine spells still have schools noted in their descriptions. The flip side of that though is that arcane and divine magic might end up "feeling" the same, and I kind of wanted to keep them distinct. [MENTION=20323]Quickleaf[/MENTION], have you run into that particular issue?