Not sure I understand the question.

The ETT figure gives me approx reach - yes, this is going to be affected by the STA and, to a lesser degree, the HTA. But as mentioned in my post earlier, the variation caused by STA is typically manageable in terms of setting up the bike to replicate a particular riding position.

Taken in conjunction with the HT length, the ETT lets me know if the dimensions of the frame are within the range that will fit me or not.

I know that if I have a bike in the 565-570mm ETT and a 175-185mm HT, I can get it set up exactly the way I want, b/c these numbers give me a fit that is right in the middle.

OTOH, if I have a bike with a 575-585mm ETT, or with a much lower HT length, then I know that the fit will require extreme measures to get it to work - and that is where STA will make a difference (a 72.5 STA bike with a 575-580 ETT may work, a 74 STA bike will definitely not).