If the boost solenoid is working properly it's 15 psi in 1st, 2nd and I think 3rd. 10psi in 4th and 5th. The overboost warning buzzer should come on at about 17.5 psi. I think it's unlikely that a stock 86 would respond well to 19-20psi, so if it's running well and you have no warning buzzer then it might just be the guage. You could easily hook up an aftermarket boost guage and find out.
You may also want to make sure that the electric fan comes on when the engine heats up. The relay that controls it is somewhat less than reliable and they can be a little hard to find and a little expensive. You can just wire in a switch if it doesn't work.
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