Your never going to fully eliminate oversteer in a RWD car, that's their nature so that's what they will want to do, to dial in some understeer will probably not help to kill the oversteer, what I recommend you do is work on your throttle control, your right finger/foot is the best weapon here, just be more gentle when getting on the power, instead of mashing the power on like you would on a 4WD car, ease it on until you feel the car is stable enough to handle the full power without kicking the rear end out