Reaserch the history of the Superfecta pay out..I think some may surprise you especially the year Supersaver won.

I like your strategy for the bet. I do a very similar bet every year.
I have a 1K bank roll for the Derby itself,this is in know way a typical bet for me I do this only for the Derby because I know there is a better than 50% chance of a nice payout.
I do a Ex,Tri and a Super bet with the Super taking up approx 40% of that 1K.
I go heavy on the Tri's as well and my smallest investment is the Ex.
I am greedy come Derby Day and looking for the score.

Good Luck and hoped I helped