A rear end flip is usually good for 5-7" of drop. If somehow your front end isn't lowered you'd have a reverse rake goin on. If I remember correctly, stock height for our trucks was about 32" ground to fenderwell up front and 34" in the rear with an assumed 225/75R15 tire, so that being said, you've probably got 4.5- 5 Inches of drop up front as it is. I personally wouldn't drop the front end any more than that without 20's to pick it back up.
Easy answer is, 6" of drop, which I DO NOT reccomend with 15" wheels. And just so you know, 3" Drop springs ride terrible. I would try and find a 1" Lift shackle set for the rear end and just bring it up.