84 300D Turbo Diesel

Just bought myself a vacuum gauge and testing it. I took a line off the main supply (off the fat one that goes to the brake booster) and hooked up the gauge. At ide it reads 24 inches and no change at higher rpm (means that the diaphram is good). 18 to 21 inches I read is the normal pressure, is mine set too high and should it be adjusted?

I don't think I have any vacuum problem at this time as I can lock and unlock all doors 4 to 5 times.

Thank you for your expert advise.