Time I reinvented the wheel, there has been a couple of previous threads on gauges and their accuracy or lack of.
I wonder about my Bravo BTP12 gauge being correct and looked at buying a stand alone 2nd gauge to check against mine.
I saw you can buy for £12 a gauge with HR fitting that covers 0 to 1.6bar or others that do just a few LB. Neither looked as much fun as cobbling something together yourself, so thats the route I took.
A gauge 0-15psi for £4 on ebay, a multi function valve adapter kit from Amazon £7 plus 2 bits of hose & clips.
Remove the schrader valve from the adapter, connect the hose and gauge and screw on one of 4 adapter ends to suit which sort of valve your boat has and your done.
Cost is pretty much the same as buying one ready made but no fun in that, also this one does 4 different valves.