Refractometers cannot be used reliably to determine finishing gravity. Calculators online and in software will get you close, but in my experience they each give me a different result and it rarely matches my hydrometer.
I use a refractometer for small batches where getting a flask full is a waste, but I only rely on it to tell me when fermentation is finished, not where it finished. I then just assume it got to about where I wanted it.
this one does seem to work best for me. you still have to use your hydrometer to figure out your correction factor but once you have that it’s pretty accurate.
I can see a wheat beer finishing in that range. But if you have a hydrometer available, I would use it and see how close you came with the refractometer and Sean’s spreadsheet.
That spreadsheet is pretty good, but IIRC, with lower OG beers, it can be a bit off. You had a plenty big beer, so it should be pretty accurate over time by entering you hydrometer readings for a few beers in this range.
Refractometers can also be good for determining when fermentation is complete as well. If readings are taken throughout the primary fermentation (a bonus since only a drop is needed), one can track the gravity dropping until terminal gravity has been reached (which can easily be indicated with a refractometer reading of no change over several days). Do keep in mind as stated above many of the formulas are not quite effective for actual measurements once alcohol is produced during fermentation, so you are really better off utilizing a hydrometer to measure your final gravity correctly.