Tire gauges are notoriously innaccurate. Either just accept the +/- 10% is likely to be as close as you're going to get, or try a half dozen different ones and use the one closest to a median value for your best shot at accuracy. Then test it repeatedly to get an idea about precision. I'm sure there must be gauges out there that are both accurate and precise, but they're probably not found at your local auto parts store.
Most TPMS sensors are claimed to be accurate to +/- 2-3%, so plus or minus a pound at 42 psi. I couldn't find anything about precision of repeated measurements with a TPMS sensor.