Feature Surface - Using Ground Levels Option

Just wanted to run something by you if that’s OK
I have a site that has a large area where we have to strip off 80mm but then add 150mm of new topsoil.
The finished levels are to match the existing levels (before strip).
 
The approach I have taken:
1.                  Strip 80mm
2.                  Create Feature Surface – selecting Use Ground Levels option
3.                  Apply an offset of -0.15m
 
My question is, is my offset correct or should I have applied an offset of -0.07m (0.15m – 0.08m strip). I’m unsure whether the software works to original ground levels or ground levels after strip when using the use ground levels feature?

Thanks