You're looking to establish the scope height?
I have the 40x and you need to have a REALLY BRIGHT light source to illuminate the paper, day light generally isn't anywhere near enough and typical room lighting is even worse. I use an LED torch on highest setting as close as I can to the paper. The tighter the spot of light is the easier it is to find the dot, as soon as the sight picture gets bright you know you're close, and your dot needs to be a reasonable size or you'll never find it.
Hope that helps
ISP Airstream MK III No 15 which is a bit poorly at the moment and having a rest
AA Pro Sport + Ginb FT8 stock + Bushnell 4200 6-24x
EV2 MK1 + Leupold Comp 40x
Weihrauch HW100 KT laminate
SBFTC Competition Secretary