Julio has great advice, set the stock wastegate setup tuner style with the hose from the wastegate to the compressor. This will get you 11 psi, maybe 12.5 if its cold out. If you have more than that pressure on the boost gauge, then look at the wastegate having a hole drilled in it, or look into the boost gauge being off.
20-25 psi on 93 octane with steel heads is way off even at 18° degrees, as in you should be around 18 psi of boost or less, unless you gotta camshaft......
Julio don-.... Making alky kits for 2011 mustang 5.0 how much?