Make sure that your cooling system is in tip top shape and that you do not have any air in the system.
I had an inop temp gauge in my 122, but was lucky enough to have a spare 544 instrument cluster on the shelf. I removed the temp gauge from the spare cluster, but before I installed it into the 122, I did a little calibration on the kitchen stove with my wife's candy thermometer and came up with the following results.
When the temp gauge needle reaches the 1st white line, the temp is at 167f.
When the needle reaches 1200 (straight up/down) the temp is at 180f.
When the needle reaches the 2nd white line, the temp is at 212f.
I installed a 180f degree thermostat, which seems to be operating properly, because the needle on my gauge always points straight up/down at 12 o'clock.
A new replacement temp gauge is still available for VP-Autoparts P/N673999 @ $216.00. There are probably other vendors who might have it for less. You might also check with Joe Lazenby to see if he has a working used gauge or check on ebay for a 544/122 gauge and/or complete instrument cluster.