There's no "formula". Depends on who you're speaking with, depends on the culture of the outfit, and lots of other things. However, facts usually work better than an emotional appeal, so look up other salary numbers for your type of aircraft, your location, etc. NBAA salary survey is a good starting spot if you can get your hands on one. The data is sliced-and-diced in lots of ways, and you can pick out the specific info that supports your argument. Also, if you have solid comparison info from other flight operations that show they are making XX more, then bring that to the table. Also, if asking for 15% or XX dollars seems like a reach, maybe ask for part of it now and a guarantee of the other chunk down the road. Also don't forget things like bonuses, 401k, etc.

At the end of the day, if they play hardball and don't budge, and you have other offers, then sometimes voting with your feet is the only way some of these operations understand the rapidly changing pilot supply / demand curve.

Good luck.