Aviation Rules of Thumb

Take feet per minute (fpm) the charts say your aircraft can climb at that given alt. and temp.
Multiply that by 60 and divide by your V_y or V_yse airspeed.

example: 1200 x 60 / 90 = 800 fpm rate of climb
tip: it's often faster to first do the last part; here: 60/90 = 2/3, then multiply 1200 by this fraction.