The rule on the amount of variation between the chorus and the verse is really quite simple and I'm happy to elaborate upon it here:
Too little is too little, enough is enough and too much is always too much. The exceptions to this rule are when too little is just exactly right, enough is just not enough, and when too much is even better. Make sure to vary your chorus just enough. Or too much. Or not at all, depending on whether your song sucks or not with it's current amount of variation.
Get a good book. Study hard and if you're lucky you too will be able write like lyrics like these:
I believe I can fly
I believe I can touch the sky
I think about it every night an' day
spread my wings and fly away

R. Kelly - Now THERE'S an artist. Fly/sky! How does he think of these rhymes!

http://www.soundclick.com/frankwilliamson