The way Tennessee's fish and game directs you to measure fish is from the beginning of the fish (nose) to end of the tail, you also need to elongate the tail fin by pushing from the top of the tail fin down and the bottom of the tail fin up.
Total length of fish should measure from the tip of the jaw or tip of the snout with closed mouth to the farthest tip of the tail with the tail compressed.
There are multiple ways to measure a fish. The most common and standard are listed below:
SL is measured snout to last vertebra
TL is measured snout to end of caudal fin (typically pinching the caudal fin together by pushing on the top and bottom of the fin)
FL is measured snout to end of middle caudal fin ray (this is how I like to measure cats because some have very long streamers on their caudal fin)