You have to have a timing mark that correctly denotes TDC for cylinder #1. If you don't have a mark and a pointer, then you can create one using the "hard stop" method - with a nylon zip-tie. Look that up via a search on this forum.
Once you know where TDC is for #1, then you can use a "dial back" timing light to determine how much advance you have at any given RPM.
Hope this makes sense!
Happy Thanksgiving and Gobble, Gobble, Gobble!
B&S