I kept wondering about the lastRPM and how it got away from zero.
Well, I thought I was doing the same code, only changing the names to my code so it would run the stepper. As to the sprintf() and dtostrf() stuff - Those are totally new to me and I didn't understand them at all. Please allow me to work on this some more tomorrow and I'll post when I get it worked thru (or stumped) sometime tomorrow