I have the same problem. I also use a homemade yardstick + tranny fluid.
I will adjust it slowly & carefully to get the levels as near to equal as poss. I blip the throttle between each small adjustment. I get it all done, am happy them open & close the throttle a few times & one of the darned levels starts to climb a bit
Maybe this small difference is insignificant as we are using a fluid with a density of around 0.8 to 1.0 as compared to a density of over 13 for mercury. This will make it VERY sensitive indeed & the difference we are seeing on our homemade device is negligible when compared to a mercury one.
I would like mine to stay put after I've set it tho
I do seal the adjuster screw thread after I have finished.