For whatever it's wort...
My own data to date has confirmed the Novotny formula for calculating SG from Brix is a good one. Not necessarily "better" than Terrill's, but a bit different, and valid in its own right.
And the most accurate FG formula for me is............
None of the above.
Actually, it's the one that Terrill refers to as the "Old Cubic", for which I forget the source but so far it seems closest on the average. Not perfect either, but close on the average.
To be fair, some of my data is for very high gravity beers, for which no formulae are very accurate -- they are all calculating FG values 0.003-0.006 higher than my hydrometer. For example there is one beer where my OG was 1.098 as measured by hydrometer (and original Brix was 23.2). Calculated FG for final Brix 12.9 and correction factor 0.99 (yes, NOT 1.04) was anywhere 1.026-1.029 depending on the formula, but my hydrometer said 1.023. So, not a big deal -- next time I make a huge beer of OG >1.090, I'll expect to have to subtract a few points from the calculated FG if I skip the hydro and just use refract. Close enough.
For me, ultimate near-exclusive use of refractometer is desirable, as I am a small batch brewer and would prefer to get 14-15 bottles out of a batch rather than just a 12-pack from too many hydro samples. For those doing 5 or 10 gallons, sure, I get why you might not care. But I do.