Without doing any calculations, that sounds about right. The calculation would be to take your current cut in speed times the new rear ratio divided by the old ratio.
PM

Is that formula perhaps inverted? I think it must be
(old cut-in speed) X (old ratio) / (new ratio)
e.g.
25 mph X 4.55 / 4.11 = 27.7 mph as the new cut-in with the 4.11 axle ratio
With a "taller" ratio of 4.11, the car will actually be going faster for a given engine/transmission output speed, so the cut-in speed should be higher.

You are right, kinda. My math was actually using the ratio as the fraction of pinion teeth to ring (like 10/40 or 9/41) so we are both right.

I found a different solution that works for me.  I bought a NOS O.D. governor for a 1950's Studebaker 6 cylinder car, based on a parts interchange manual I have.  It says the only difference is the engagement speed, and I took a chance that the Studebaker 6 would need a higher engagement speed, based on experience with my parents' 1959 Lark VI in the 1960's.  The Lark VI was a 99 horsepower wonder, and I didn't think it would have the torque to go into overdrive at so low a speed.  This was a SWAG, since I also didn't know what rear end ratio the Lark had, compared to my Dana 4.09.  I determined that the Studebaker governor cuts in at 30% higher RPM than the one that came with my Hornet, so that increased the engagement speed from about 22 MPH to about 28 MPH.  I'd prefer for it to be even higher, but that's improved enough to satisfy me for now.

My backup plan, had this not worked out, was to experiment with one of the governors, knowing that I had the other as a backup.  But that was not necessary, at least, not for now.  And regarding the original speed at which the governor was supposed to cut in, I have read various sources that say 26-28 MPH, and others that talk about 18-22 MPH.  I think the intention was that you put your car in 2nd gear and just leave it there around town, letting it start in 2nd and then go into overdrive 2nd, which is close to non-overdrive 3rd gear.  When you slow down, it goes back into non-overdrive 2nd.  But this use doesn't interest me.  I don't mind shifting -- if I had wanted a Hydramatic, I would have bought a Hudson that had one.

I have the Studebaker governor part number, if anyone is interested.  They are readily available NOS.