I think we did find some research supporting the idea that higher concentrations of topical testosterone on a smaller skin area generate less DHT than lower concentrations on a larger area, with the same total dose. This is because of the local saturation of 5-alpha reductase.
Even without scrotal application, topical testosterone can be very efficient at raising DHT. For example, on Androgel with a total testosterone of only 165 ng/dL, my DHT was measured as 65 ng/dL. A little while later, on injections, total testosterone was 1,250 and DHT was under 50 ng/dL.