Any reason you don't do squats instead? What is the benefit of standing from a seated position?
I don't know if I can count the hand squeeze reps as 500. I understand that you did them (not sure of the resistance difficulty), but counting them as one rep each just doesn't seem fair for the competition. I'll say 150 reps for now. How long does it take you to do the 500 total? Both hands at the same time?
I have used hand grips many times over the years, I think it would be fair to say for 500 would = 100 reps. They do get tiring and work the hands and for arms really well. But they are not as strenuous as a half hour run and could smash out 500 in like 10 mins easy.
But 150 reps would be a good call too m8 :)