Before tweaking anything, make sure all the pivots and lock points are perfectly clean. If they are, then yes, I'd first be looking at the tabs you indicate. It's a pretty simple mechanism, which makes it easier to see what effect any changes you make are having overall.
The measurement point is a good one. The way I've done it is to remove the lens and shutter assembly, then use a plunge micrometer from the shutter flange to the pressure plate (back closed, obviously - you may want a single layer of tape on the pressure plate to prevent scratches). Measure at four cardinal points around the flange so you have a clear indication of what's going on. It's not as precise as a bench setup, but you should be able to get within 0.10mm pretty easily.