Tomorrow i am installing my HKS RS coilovers. I want a clean no finger gap drop with my 19" 35 sidewall rims/tire setup. I was wondering if anyone had any measurments for me to use as a good base to start from, so i dont spend all day adjusting them. Much appreciated.

Youll need to adjust for your tire size vs oem tire size, but here are the oem coupe ride heights on 18's.

Oem ride height is 27.32" front
Oem ride height is 27.64" rear

You're gonna have to adjust them to get them correct anyway. I would install it, drive the car around the block, measure, then adjust. Most likely they will settle just a tad too over the next few weeks.

Have a set of H&R coilsovers on my other car. Like most have said they are going to settle a little anyways so you are going to have to adjust them later on. For now just make sure that both front and both rear match each other as far as the position of the height adjustment rings in relation to a uniform spot on the schock. This way you save yourself from having to adjust height separately for four different coners instead of just the front and back. Have fun.

Is this the fenderwell gap to the ground measured in the centre?

Yes, on 18 wheels, I dont know what is on 19