Please don't do the leg extension exercise machine. It's one of the worst if not the worst exercise you can do for your knee. The bottom position of it puts a lot of stress on your ligaments + it is not "functional" (hate to use this word) for snowboarding at all. You're better off doing barbell squats (until parallel), deadlifts, romanian deadlifts, 1 legged DB romanian deadlifts, glute ham raise, reverse lunges.
I agree (if you notice my previous page included the 1 legged deadlift and other body weight exercises)... but I don't think doing a few reps in one day just to get an idea of what your quad/hamstring strength ratio is going to do any serious harm.
You don't need to do max weights... just do something where you can do like 12 reps at medium exertion on the leg curl and and find what weight is roughly the same exertion level on the leg extension.... or just do a lot of hamstring exercises and quad AND hamstring stretches. I've never found anyone with overly developed hamstrings (compared to quads) so you really don't need to measure it I guess.