s13 sr20det pulling 10 inch of vacume? problem?

I recently yanked the ca out of my s13 and droped in a sr20. Everything seems to be fine accept for vacume.
Boost guage is telling me im only pulling 10 inches of vacume, shouldent vacume be closer to 20?
I was thinking a vacume leak but there is no fluctuation where iv seen leaks go from 20s to 10s and jump around.
Any one have any pointers on where to start?
Right now its running stock 8psi from waste gate with no boost controler.

this could all be cause by a vaccum leak even if it dosnt fluctuate. All you have to do is give it pressure true ur maf and see if you can here any leaks from anywhere.

an obvious vacuum leak or maybe a faulty gauge I've seen both cause this