Well, the "Size A powerplant always uses 2 tons of fuel (for a month)" is literally High Guard rates expressed in a different form. If it's by letter rather than output, the TL-15 drives (W-Z) also get a fuel-efficiency bump out of it.
The problem with just going to 1%*Pn (or, in this formulation, about 1 ton per EP) is that it enables hot maneuver drives in small ships. I'm not totally averse to this, but it's going to unbalance things. I'd think it ought to be more in the range of 5 tons per letter (2.5 tons per EP) to keep some degree of forced trade-off.
The key points are to switch to a formula that's proportional to both rating and ship tonnage rather than the existing rating-only formula, and to keep the fuel requirement high enough to place meaningful limits on M-drive capability in smaller ships. That would then leave the fixed-size 20Td bridge and the computer as the drivers of scale efficiency.
Others here consider it to be outright heresy, though.