Normally I don't recommend plated bullets for new reloaders. There seems to be not enough easily found data/info for loads and methods and new reloader are often confused. I recommend starting with jacketed bullets, the easiest to reload, and then branch off to other types after you feel confident with your reloads. (hint; find a load, bullets and powder, in your reloading manual before you buy components)

I'd say don't bother with any "formula" and just use lead bullet data. Two percent? 15%?, don't bother, just go with listed starting loads and work up (if you look at forums long enough you'll see a formula for anything) When you get a lot more experience and knowledge, then you can extrapolate data if you choose (I've been reloading for 30+ years and don't bother with "WAG reloading data").