3top: I'm not sure you should try the 30kt auto, even from 600 feet, and especially at 500 fpm rate of climb, in the R22. If there are no further posts here, I'll know you've tried it.
The 90 per cent gives a good margin for error, but what troubles me most is the rate at which the needles were heading south when they crossed the line. I had an old instructor, now dead of natural causes, who had 17,000 hours' rotary going back to 1948, who once took the Nr down to 83 per cent in the cruise in an R22 and commented helpfully that he "didn't think it was ready to quit yet". I didn't fly with him again after that.