Start the engine. Pull the vacuum line off and plug the line. The fuel pressure should read between 41-47psi. Your manual is 100% correct. The injectors are flow rated at 3 bar = 43.5psi. If you don't have a fuel pressure close to that, they will not flow the correct amount. 38psi without the vacuum line is not an acceptable fuel pressure.

Put the vacuum line back on, with the engine still running. The fuel pressure should drop proportional to intake manifold vacuum. With a stock cam, that could be in the range of a 6-10psi pressure drop. With a more radical cam, you might see only 1/2 of those numbers (3-5psi).

Now... tape the gauge to the OUTSIDE of the windshield, and take it out on the road, and run it up to max RPM/WOT. Fuel pressure should stay within 1-2psi of the pressure you measured at idle with no vacuum.