A little crude testing with a 50 lb. scale indicated I needed about 20 lbs. of pull while exerting pressure against the half-nut with the handwheel. I found my spring at True Value, but this is the same.Good point. Oil is constantly being supplied up high and it slowly flushes down into the sump.
It sounds like you have played with different spring tensions, any chance of providing a source/part number for the spring you ended up using on the retractor arm?
So far, I've only tested up to 3/4-10. At coarser pitches, the pressure against the half-nut increases, both due to the greater travel per rev and the larger face contact (=chip load) nearing the finish. Planning to try 1-8, which is as coarse as my threading inserts go.