What I did was use Play Now to work out the sliders before you start the franchise. With no hot or cold streaks, you should get very close to accurate stats, such as throwing 50% completions with a mediocre QB against a good pass defense, or getting 5+ yards per carry with a good back against a shoddy run defense, etc. So once you have the sliders where you want them for realistic numbers, you start the franchise and when someone has a bad game you know it's because of a cold streak and not because the sliders.
See my slider thread here:
Robcards' All-Madden Sliders