It's been a few months since I've weight-mapped anything, but off the top of my head this is my typical method...

Select the joint that you want to smooth scale. In the joint editor dropdown for that joint you should see some SmooX or SmooY entries (depending on the joint order/orientation), usually there's a couple such entries relating to the smoothing/transition zones between parent or child of said joint. Basically the affected actors...

A bit of explanatory divergence for a second; If you click Edit Scale Zones it will show a Hierarchy and the affected actors of the Joint Scaling. This should already be checked in the Hierarchy. Similar to creating Smooth Translations you could add affected actors to the joints scaling from here by checking additional grandparents or grandchildren (guessing here, as I only ever create smooth scaling between the typical parent/child relations, which should be already present)...

Anyway, I choose one of the scaling zones from the dropdown (shown above, first illustration) and go to the Falloff Zones: Add dropdown and create a weightmap for that joint, setting it as replace via the smaller dropdown arrow directly above that...

To be honest, my workflow is to manually add weightmaps as I go along set as replace in the joint I'm currently working on. I don't usually merge to.

From here edit the weightmap's scaling influence via the brush settings, for smooth scaling you'll generally be using the smooth brush and very little of the add or subtract...