Dbate,
Your equation is correct to onvert the gradient value from ft/NM to Percantage or vice versa.
Example 300 ft/NM gradient is eqaual to 5% (=300/60);
Or
8 % gardient equals to 480 ft/NM (= 8 x 60)
These equations can easily be derived because 1 NM equals approximately 6000 feet.
And one more small thing: For small angles, 1.7 % gradient equals approximately 1 degrees (Because Tangent 1 degree equals to 0.017). So If you have FPV ( bird) on the EADI, you can read your instantenous climb gradient.
Regards