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
2010 FLTRX , Stage 1, 13" Madstad, Mono tubes, SuperShox, Heritage style bars. Alpine CDE-HD149BT,Soundstream Picasso Nano Series PN4.520 , 5X7 and 6.5 Infinity Kappa speakers
If I have to explain ,you wouldn't under stand anyway.