I believe 3/4 is full boost; gauges are typically designed to read within their upper and lower limits under normal conditions.I also noticed that my Turbo gauge does not go all the way at Full Throttle. It only goes up to 3/4 or something.
I think it should go all the way to Max when at full throttle !
Is it related to the vacuum leak ?
It’s possible for a vacuum leak to affect boost, but IWE leaks are typically after the check valve, so boost should not bleed off.