First, different labs have vastly different ranges and numbers. So you cannot easily compare or define such things in some cases. Quest FT has very high numbers.
FT near or at high range for old guys seems to work well, but many will not get there when their TT is limited to normal ranges and docs will often not want any labs exceeding normal high range. Younger guys will get more benefit from a given FT than old guys.
And FT:E2 is probably good metric as long as E2 is not so low that adverse things start to happen. So FT=30 with E2=30 would be 30/30=1.0 and FT=30 with E2=20 would b e 30/20 = 1.5. There is not real science behind that and I just made that up, but it does have merit. Similar to some androgen indexes that factor in SHBG. Note that more E2 typically increases SHBG and more FT or bio-T lowers SHBG. And also note that E2 blocks some of the molecular action of T at the cell walls. Both T and E2 affect gene expression inside the cell nuclei and the effects of one can also involve inhibiting the effects of the other.
SHBG is a big factor. SHBG is thought to increase with age. In many cases that would result from guys simply getting older and fatter with T levels that are also dropping. But in a TRT context, there can still be some odd things going on in time when T levels, E2 levels are maintained the same over the years. In my case, with 100mg T per week I started off with FT around 35 [Labcorp]. With no weight changes and waist size not changing, my FT levels dropped over the years and T dose was increased to restore some of the energy and libido benefits of the past.
Another issue is that labs have age brackets for ranges. Suddenly your high-normal range T levels can become high above range after a certain birthday. Many progressive TRT docs want to restore T levels to youthful levels. Ranges that decrease with age are contrary to that intent.
When there is a problem with higher levels of SHBG, TT levels become exaggerated vs FT levels. In such cases, TT should be driven above range to get FT to high normal. Most doctors simply do not understand the implications of T+SHBG been non-bio-available. In that case, bio-T would be the best target, but few use that. Higher FT may then drive down SHBG over time. Note that there can be other medical causes or drugs that affect SHBG and the liver’s ability to remove E2 from the blood stream that then increases E2 which can then increase SHBG. So functionally, there is a lot of potential complexity and variability and we seek simple metrics as guides. Sometimes we need to see when one needs to change the game.