It'd help to know what kind of fuel do you use? What mods do you have? Stage 1,2, 2+, 3?
For example in my stage 2+ Cupra I run with 9 boost, 5 timing, 9 fuel., before the intercooler upgrade i ran with 9-4-9, at stage 2 I ran 7-4-8, etc.
For a stage 1 GTI i belive u can run 7-6-9 with good pump fuel.