Perfect dose is 10mg to start, youll get high very very minimal risk, oxy i find ( active ingredient in percocet) is on the low side of respiratory depression ( what kills opiate users )
I'd say it's pretty much impossible to dose on 7.5mg oxycodone. If you weigh under ~130lb I'd take the 7.5mg, otherwise I'd take two (15mg) or you'll probably be underwhelmed.