If I do up the straps to a comfortable tightness I can't reach the fuel cock.
I fly aerobatics with a five-point harness and there's a little knack to tightening them properly.
What you need to remember is that the primary function of the harness is to keep your pelvis firmly in the seat during any type of maneuver. A secondary function is to keep your upper body restrained during a crash.
So you start with the bits that form the lap belt. These need to be done tight. Second is the crotch strap, if present, which also needs to be done tight.
Only then do you need to do the shoulder belts, and these can be relatively loose. One or two centimeters on the "loose" side of "snug" is typically enough. If you're not sure, you can always tighten them upon landing. But I've never had the need for really tight shoulder straps during aerobatics.
When worn this way, I can reach each and every control, knob and lever without difficulty, even when inverted. And for a PA28-style or C172-style fuel cock you can easily slip off one or two shoulder straps, move the knob and slip them on again, without needing to adjust them.
I have often seen people in four-point and five-point harnesses which were done up improperly, leading to the situation where the buckle was at the level of their sternum. Very uncomfortable, and incredibly dangerous during a crash.
