Rule of thumb
As this is a simple relation between gained height over travelled distance you can easily express it in an equation and build your own rule of thumb:
Gradient (%) x 60 (precisely it`ll be 60,75) = Gradient (ft/nm)
eg. 8 (%) x 60 (factor) = 480 (ft/nm).
Then just look for your avg. GS during climbout (~V2 +-). Each 60kts of GS equals 1Nm/min, hence if you go at 180 you`ll make 3NM/min and therefore require, with the
above approximation 3 x 480ft = 1440 ft/min. Since you only approximate the factor, put a little safetymargin on top and end up at 1500ft/min.
Cheers BG