This type of problem can be solved using the fact that
% Climb = approximately (ROC in ft/min) /( TAS in Kts)
This can be rearranged to give
ROC = approximatley % climb x TAS
To use this equation you ust use your CRP5 to convert your IAS of 85Kt at a pressure altitude of 8000ft with a temperature of +15 C, into TAS. I do not have a CRP5 to hand, but you should get about 100 Kts TAS
Putting this together with your 3.3% climb gradient into the equation gives
ROC = approximately 3.3 x 100 = approximately 333 ft/min.
If you wish to be more accurate use the equation
Still air % gradient = (ROC/TAS) x (6000/6080) %
You can find this equation in your CAP 698.