Calibrate your hygrometer. Only takes about 8 hrs to do. Will show you how far off it is.
Only way to get 52% RH in a dry incubator is if the humidity in the room is higher than 52%. Or you have a room temp of 99 degrees and have 52 % humidity in the room.
The incubator will pull in room air through the vents... (they are open right?). It will then heat the air up to incubator temp... 99.5 degrees (or thereabouts). When you heat air, it's relative humidity will go DOWN. So to end up with 52% RH in the bator, the room RH % would have to be higher... since I'm guessing your room isn't 99 degrees.
For instance, my incubator is kept in my temp and humidity stable basement. Temp is 55-57 degrees and RH % is right around 50% all the time. My dry incubator relative humidity is around 27% (hygrometer calibrated).
Be aware that room RH% will fluctuate with outside temperatures... and with warm/cold fronts coming through. So you need to have the ability to add water should the humidity drop.
That said... I would not worry about incubating the whole 21 days at 50% RH. Many have done it, especially when staggering hatches. I'd candle the eggs at day 10 and 18 to ensure the air sac is looking the right size. Air sac too big... increase humidity. Air sac too small... decrease humidity.
Good luck!