Benbow,
I used to do the same and this is preferred but if you glass slips at all, then it messes it up. For template, the best thing is something flexible like a another piece of glass or even pexiglass. What I started doing is grinding the limb flat on one side on my edge sander so I knew I had a good flat square edge and then align the pattern up flush with this and clamp with spring clamps then trace. The way Benbow described is best but I need to make some adjustments to my form so that there is no chance of the glass slipping during glueup