Mine goes from 12psi at 140 to 20psi at 180 so you might be normal, my high limit is at 180 and I haven't the need to raise it yet. The factory setting was at 210 and that wasted a lot of coal for no benefit.
You should be keeping a 50 degree spread between your low and high setting if you are talking about your efm boiler. In the warmer months, your boiler temperature may drift above 180 and neither your timer nor your thermostat will allow the stoker to start up, resulting in the fire going out. This is because the pot is still hot with burning coal after the stoker shuts down and it will continue to heat the boiler's water. With no circulator or domestic hot water use, your boiler temperature could rise 50 degrees after the stoker shuts down.
Your low limit setting, which is normally 150 for cast iron and 160 for baseboard is what controls your boiler. The boiler will maintain the low limit setting when it is resting. Upon a call for heat, the circulator will start up, cool water will enter the boiler and the stoker will start up as the boiler temperature drops below the low limit setting. The circulator will draw down the boiler temperature about 10 to 15 degrees (differential setting) below the low limit setting and then shut off while the stoker keeps fueling the fire to bring it back up to the low limit setting. This will ensure that there is hot boiler water available to heat the domestic hot water coil. Without the differential setting feature of the triple aquastat, the boiler temperature would be drawn down to a point where you wouldn't have any domestic hot water. After the room thermostat is satisfied and the boiler is at least at its low limit setting, the stoker will then shut down.
The high limit is just that...a safety limit. It won't allow the boiler to go above that setting. Leaving it at 210 will not cause any more coal use than if it was 180, but it will prevent your fire from going out.
This would be a typical example of how the aquastat works: The boiler is resting at its low limit setting of 150. The thermostat is turned up and the circulator brings cool water flowing into the boiler. The boiler temperature drops to 149 degrees and the stoker starts up and tries to keep the temperature at 150. There is too much cool water returning to the boiler and the stoker can't keep up. The boiler temperature is now at 140 and someone is taking a shower. The aquastat shuts down the circulator so that the boiler water doesn't get any lower than 140, so Mom can finish her shower. Meanwhile, the stoker is still working and the boiler temperature starts climbing again towards 150. The circulator starts again and the cycle continues until the boiler water temperature reaches a stable 150 degrees and the thermostat is satisfied. The pot is still red hot and the boiler water starts to climb, as much as 50 degrees in the Summer. That's no problem, because the high limit is set at 200. In a little while, the stoker timer will run through its 3 minute per half hour cycle and the fire won't go out. If the high limit is set too low, the timer will come on, but the stoker won't start up because the boiler water temperature is above the high limit. A half hour later the timer cycles again, but the fire is now out and raw coal is fed into the pot. This can be prevented by another aquastat which tells the stoker not to run because the outlet pipe of the stoker is too cold, indicating an outfire. But that's another story.