The VG's should come with a strip that lays on the surface that clearly depicts the VG placement. Trying to do this and get it right without the pattern, in my opinion, will not work. What kit do you have?
I don't have a kit. I have an experimental and wanted to "experiment". I thought that if I copied someones, it would give me a starting point. I only want to try it on the stab for now, as I do loose elevator authority prior to the wing stalling.