It seems impossible to set rules about rpm's vis-a-vis HST etc. I've heard so many different and conflicting ideas on this. Terrain matters, speed, gear, FEL load weight, and lots of other variables. Ultimately each owner must develop a "relationship" with the engine and just feel out and hear out the ideal rpm, which of course constantly changes as you work. I don't like over-revving any more than I like lugging. I guess those are the only hard rules.
My dealer, when I bought my CK35 HST, said 1500 was the ideal engine speed when just driving on level ground. But I've come to feel that's too low and don't usually do anything on the tractor at 1500 except start it!
Who knows.
P.S. I was moving a huge pile of old, wet, heavy mulch yesterday and I goosed up the rpms to 2200 when scooping and that seemed just right. Maybe I've been babying the engine too much (of course, babying is lugging if it's overdone). Still just getting to know to my tractor---have only 50 hours on it, after all.