your right... using this calculator
http://meadcalc.freevar.com/ with just the 12lbs of honey in the 5gal must you will have an approximate brix of 20.7 with a potential ABV of about 11.5%
Adding 4lbs of blueberries the brix is about 21.44 with ABV of 11.91% (but it sounds like you already know this?)
Did you calibrate your refractometer? Distilled water should read zero (0)
Is your refractometer an ATC model (compensates for temps) if not, the following links to sites for calculators can help with temperature correction as well as brix readings from fermenting must (current and final gravities)
Refractometer results are only accurate before pitching the yeast, once the fermentation has started you must use a refractometer calculator to have corrected results.
the one here is very accurate but requires microsoft xcel (scroll down to spreadsheet download)
http://seanterrill.com/2011/04/07/refractometer-fg-results/
if you don't have 'excel' try this one, not quite as accurate but will work if you don't have a hydrometer.
http://www.brewersfriend.com/refractometer-calculator/
USE A WORT CORRECTION FACTOR OF 1.0 ON BOTH CALCULATORS for mead or wine
I have used both and find mixed yet similar results compared to a hydrometer but I'm a new brewer and have a cheap hydrometer and refractometer
A hydrometer would be your best bet! at a minimum you could at least verify, confirm or correct your results from your refractometer
As far as having a dual scale refractometer...you would still need to use a calculator after fermentation has started so a brix scale is fine.
Hope this helps!
If I've spoken on what you already know, sorry