Sounds OK, l might need to stat up a couple characters to see how it works in a mock combat
The +1 per every 5 ranks of best melee was already part of the figuring of the DT, for many versions of the manuscript before this one.
So the only change to it is that pure arms users would get +1 every 4 ranks rather than +1 every 5 ranks (in either case, the bonus tops out at +5 total).
And that pure spell casters would not get it at all (their bonuses were always smaller than those of the other TPs due to their focus on spells over arms).