Maybe this is obvious to me because I use hand tied leaders, but I've noticed a lot of confusion like this lately
When you buy a store-bought tapered leader, you should always add a tippet to it ... first thing
Say you buy a 7.5' 4x leader, and you add 18" of 5x tippet. You'll end up with a 9' 5x leader.
If you do this, you'll always know when your tippet is short and needs replacement
On top of that the original leader will last much longer as you'll never be cutting off more than 2" at a time whenever you change tippets
And if you use a loop2loop handshake for your connection, you'll never be cutting back the leader ever.
As for measuring tippet length, ...that's really not important. As long as you have 'enough' for a drag free presention