While you are doing all of this

1. Flush the tank if you have a way to. This should be regular practice if you had it or if it is plumbed in a manner consistent
2. Per the post above Read about "Anode Rod Replacement". You should plan to do that too. It's easy given what you are about to do and who wants to disassemble in a year or two for it.
3. Ensure the burner flame is proper. There are videos online that show how the flame should look. Just because it is lit doesn't mean it's working right

There are good posts on how to measure the length (counting wheel, etc). In terms of getting tension consider doing what many commercial sewing machines and almost every motorized vehicle does with it's belts.

That is you change the direction of the thread so that it creates tension across the surface of whatever you need to turn to get an accurate measurement.