Convert Climb Gradient to Climb Rate (RoC) --
-- Three Degree Glideslope
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 Vy or Vyse 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.