Take your time and look at some other bikes to see how far back you want it. After you decide positioning, use some two inch masking tape about where the legs will mount .Use four pieces of two inch by two inch . Now all ya have to do is trace around the legs on the masking tape and drill.
I also had my bike standing straigt up when positioning the rack
10 FLTRX , Stage 1, 13" Madstad, Mono tubes, Heritage style bars
If I have to explain ,you wouldn't under stand anyway.