Oh, btw i know what to add to my post
if you want to be efficient, you should be focused on making one task after another, if you can manage to not make to many things at the same time you can be very good at ecoing,
If you have two things half finished it can be analysed as a mistake, cause you could finish one thing 1rst and start getting value from it, and then do other thing, and addictional value of 1rst thing, while second thing is in construction will give you small advantage, in every stuff you do
TA4Life does that with his 1rst engyneer almost every game, he just assist acu to make pgens/mexes faster so he gets small bonuses for that, however other players just send engyes for expand are not doing it wrong, wrong would start other thing with engy on your base while thing that acu makes is not finished (not the case of t1 extractors if they are to far away from acu)
Phantom mod is even much better example if you want to see player ecoing...
in regular games i try to stuck many engyes in one place and i make some facs some pgens, and amount of engyes ofc limited by pathfinding issues and bomber run possibility, but i knew that idea of efficiency by making one thing after enother long time ago, and that is why i like to make some facs and some power with
same engyes, not sure if it is very good, and sometime i suffer from bombers, but it at least gives me feeling that i am efficient
so basicly you can build your eco and stuff that way so you upgraide mex one by one, with assist or not, (but not many mexes without assist), but keep your eco/build power balanced and you will be in great shape.
OFC youre limited by engyneer movement, so mexes that are hard to reach can upgraide themself...
but you can see players like Yama_Dharma just balance eco well and have really good results
also i personally send extra engyes on reclaim instead of assisting mex, and start more mex upgraides at the same time
i think every good player assist his mex time to time, some profere assist t2, some prefer assist when they get t3...