I was given a formula back when I calibrated it. It was quite a few years ago so I can't remember what it is now. It had to do with the size of the load in watts indicated on the product, the Kh, the number of revolutions on the load wheel down to the fraction indicated on the load wheel at the end of the course of one minute on that load, while running everything on 120V. I'm not sure whether you do the LL of the FL first, but for the LL adjustment I used a 25W light bulb, and for the FL adjustment, a 1.5kW space heater. I was going back and forth between the two products until I got the right reading on the load wheel for both until no further adjustment was needed.
You'd basically be looking for the RPMs the load wheel needs for a certain size load, comparing it to the actual RPMs you get, just as if you had it on the machine. Then you would adjust accordingly.
That was before I found out that you could put the output of a small isolation transformer in series with the line on neutral to get the B leg that energizes the field coil for a 240V meter. You just simply don't connect anything to the output on the B leg and use anything on the A leg like you normally would.