Look at it from this point of view...
How hard (in kg) could a 20 kg tuna pull before the line snaps.
So your hooked on and the 20 kg tuna takes a hard charge what kg line would you use as a minimum on the verge of broken off.
If you use lock-down drag, your point of view is relevant. However, as we use drag for tuna, how much drag you like to use is more relevant.
For 20 kg tuna, I like to use 15 - 18 lbs drag and the line I use should be 50 - 65 lbs accordingly as the max drag I use is 1/3 of line rating.
You can fight a 20 kg tuna with 10 lbs drag and 30 lbs line, but you need a reel with more line capacity.
The problem with Yellow Fin school tuna is they can be anywhere from 30 to 80# with a few over 130# mixed in here in our Gulf Waters. If you plan your gear on 40-50# Tuna using 15# of drag and get one over 100# you are going to be in for a really long fight!
It's best to use at least 65# Braid so you can use 20# of drag and use an 80# FC leader for chafe resistance.
Remember... You don't get to pick the size of the fish!