Hi Tim, glad to read that your efforts paid off and your speedo is working to your satisfaction. Changing the distance between the spinning magnet and the drag cup will indeed change the attraction between them which will change the MPH pointer. Have a look at the thread in this link. You might find it interesting to read about a different way to achieve calibration that involves changing the strength of the drive magnet. It doesn’t take much and the results are reliable each time. Unfortunately, now that you’ve changed your diff to 3.07, your odometer will not be accurate. You need a speedo that has a TPM of 1120 for that diff. Calibrating the odo section requires a gear change in the odo section. And these gears are not easy to find. I’ve read reports from other listers that some of the speedo shops will calibrate your MPH display but not your odometer.
When I changed my diff from 3.54 to 3.07 I was changing it from a 1312 TPM to 1120. I figured the gear I needed was out there somewhere and I eventually found it at local car flemarket when I stumbled across an old 1120 speedo from a sedan that I bought it to harvest the gear. As a result, I have effectively converted my speedo & odo from a 1312 TPM to an 1120 TPM. it takes a bit of doing, but with the interchangeability of all the parts in these speedos, you might want to get the odo calibrated at some point.
Good luck
Anthony