What I would recommend doing is remember your TD's fuel prediction from the last race and check how much he was off per lap, and adjust accordingly for the next race. For example if he predicts 4.5 litres per lap and it actually is 4.9 liters per lap on a specific push level, adjust that for the next race. When he predicts 3.4 for the next race just assume 3.8 litres per lap. Has almost always worked for me.
I have noticed with the new update that my Tech Director is very very close to what is pirdicted prior to the race. 3.6L / lap pre-update was really 4.0L per lap an now Its more like 3.8L / lap post update an on full push I am using 3.9 -4.0L. This also varrys per track an what ever partners your using I am assuming, but this is what I have seen an I am very happy with this change.
Does the Technical Director just inform you of fuel usage and recommend pit stop numbers for a race? Do they have any influence over any other part of the game?