Determining coilover ride height?

I understand that this may be in the wrong section, but I am bringing the car to the shop this Saturday to have the height adjusted on the coilovers, and needed some quick insight. 2008 GTI with Bilstein B14 PSS Coilvers that are all the way low on the car currently. Now I understand I have been riding on the internal bump stops so this is why I am raising the car.

Bilstein contacted me back and recommended 20-30mm lower than stock height in the front and 10-30mm lower than stock height in the rear for OPTIMAL settings. So I asked how many spins I would need to perform to get to 25mm drop in the front and a 20mm drop in the rear, and they told me to put the stock suspension back on and measure it and then determine that way.

So my question is, how do I determine to adjust the coilovers so that I get a 25mm drop in the front and a 20 mm drop in the rear? Will my mechanic know how to do this, I just want to be prepared with as much information as possible so I can finally dial in my suspension!

18X8 Wheels with 225/40/18 tires. I also understand that the rear adjustable end links will need to be altered with the change in height as well.

Thanks for any responses.