I'd do what Geno suggested. It's the clearest and most cohesive way to write your lyrics. You should definitely check that link in my signature as well.
Writing in couplets is also good because if you have different sets of ending rhymes on every 2 bars it'll keep the whole thing more interesting. As for the "mathematics" of it, just do the first line that ends in a rhyme and it's a clear statement. Like "I'm sitting here typing crazy advices". So what you do is, you think of a couple of rhymes for "advices" (or maybe for "crazy advices" if you want to do multies) and then pick some of the rhymes that you like. See what that word is associated with i.e. what does it mean. For example first thing that came to mind for "advices" is "vices". Vice means a bad habit so I can say:
I'm sitting here typing crazy advices
Drinking and smoking are my vices
Now this is stupid as fuck but you know what I mean. Or if I opted out for devices or dices, I could've said something else and direct my ides in a different way. Also try to keep the length of each line around the same as the previous one.