Do your own tests. Fly a series of climbs at different IAS; note the ROC for each climb. To be all pure, make note of OAT, pressure altitude etc. as well.
Enter IAS and ROC points as x-y pairs in a spreadsheet.
Then convert each IAS to ft./min. so speed units are the same as ROC.
Then divide each ROC by its associated IAS; you get a series of climb gradients. find the most favorable one, and the associated IAS is the one you want.
You can even plot the points (gradient vs IAS) to make a pretty graph.
