What PTO RPM is best for a round Baler?

I'm relatively new to the hay business. Have been producing my own hay now for about 5 years. I was baling this weekend and this question popped into my head as I watched the tractor fuel meter steadily fall - what is the best PTO rom to run a round Baler at? I've been running my Deere 467 at around 500 PTO rpm which requires the engine rpm to be about 2000. I bale tif ton 85 and Jiggs.

What PTO RPM is best for a round Baler?