Hey brother, I think I have you covered. I have a '62 M20C with bladders and I just did a custom Fuel Stik for my plane. I used a siphon plus a 2 gallon gas can and measured out two gallon increments to get accurate numbers. I don't have an engine monitor but have tested the burn based on power settings over 10 flights since measuring and the numbers come out within about a half gallon or so of where I predicted. If you'd like to buy a stick from Fuelstik.com, Sportys, Aircraft Spruce, etc., then you should be able to use this scale.
By the way, I cleared up a bit of a mystery when I was working on this project about the true fuel volume of the bladders. My '62C when built had only 48 gallon tanks according to the owners manual. My aircraft and STC records only reference bladders added and their serials without stating the fuel capacity, so I reached out to Griggs who confirmed that my tanks were indeed expanded to 54.8 gallons total with the bladders because that bladder 27.4 gallon bladder (each) size corresponds with their part number. So now I know for certain that my tank capacity is indeed higher thanks to the bladder STC. I cannot recommend Griggs enough based on the level of help they provided in pursuit of answering my burning question. I will not hesitate to work with them if needed in the future.
Printable Mooney M20C 27.4 Gal N6475U 6-16-23.pdf