It all depends on how fast you can spit the lyrics, if you're Big Pun or Twista, then probably 3 lines of text in a bar, it all depends, that's why the structure in some rap lyrics don't look very neat and tidy like verses written specifically for text. I think the best way to solve this is to abolish talk of bars at all, and just mention the amount of lines involved, that takes away the confusion.
P.s No prob C-Game, I see what you're sayin, but it makes the verses really shit doin it like that. Sometimes it's really cutting off a good rhyme flow to edit it to six lines. We'll see what happens in the coming months.