The coolant system pressure will change based on the temperature of the coolant. As the coolant heats up, it will start to generate steam, which will pressurize it, therefore increasing the temperature needing to boil it. It will continue to do this until it hits 16psi, at which point the cap is supposed to vent the coolant.
So, the pressure will vary depending on coolant temperature. At 190, you'll probably see a little pressure, but once you hit 210-215+, it should start building rapidly. IIRC, the boiling point of coolant at 16psi is about 250ish, so you should not see 16 PSI until you hit about that coolant temperature, after which, it should vent out the cap.
The pressure itself is what keeps the coolant from boiling, which is why, if anything in a coolant system gives when hot, you get RAPID conversion from liquid, to boiling, to gas. The leak causes the pressure to drop, which lowers the boiling temperature, which causes more boiling, which causes rapid expansion of coolant and the blow-out.
Where are you measuring the pressure? That makes a difference.
If you are seeing 16psi before your ECT sensor says you hit 250, then your coolant is boiling somewhere, away from the ECT Sensor, adding pressure to the system. The ECT sensor is right below the thermostat. It is possible for the Coolant near the ECT sensor to be one temperature, and coolant returning to the water pump, or going elsewhere in the engine to be a completely different temperature. Pressure is funny like that, a hot spot somewhere in your engine can raise the entire system pressure, without raising the entire system temperature, since it takes time for that heat to disperse. Your water pump may not be moving coolant as well as it should, you could be having hot pockets of flash boiling in your engine channels, you could be having cavitation issues, etc.
So to answer your question, anywhere from 0-15 PSI is an acceptable range, depending on the temperature of your coolant. Don't worry about the pressure so much, but rather, whats causing it. If your coolant is too hot, if your coolant is flash boiling somewhere, etc. Pressure is a byproduct of heat. You have to examine the source of the pressure, not the pressure itself. Check your coolant, make sure it is 50/50, if you're running a 50/50 mix. Too much water will make it boil easier inside the engine channels.
It's things like this that make me even happier that my coolant doesn't boil until 370 degrees, and runs at 0 pressure
Street tunes are known to run the engine leaner & hotter, which means higher cylinder wall temperatures, which can cause hotter coolant in the channels. I do hope you're not attempting to Tow on your street tune. That is definately not recommended. Street tunes are for unloaded driving, if you're towing anything more than a few thousand pounds, you should either go back to stock, or use a Tow tune specifically designed for towing.
25psi is not overboosting if it's holding it steady. Overboosting will generally surge and be unpredictable due to VGT issues.