I actually don't have a house rule, but the one thing I always wanted to implement was a one-ton granularity for jump drives. Such that you didn't have to design ships that were 100 tons, 200 tons, 300 tons and so forth, but could design ships that were ... 112 tons, or 284 tons and so forth. I think it would free up people's imaginations to design more imaginative vessels.
Already ahead of you (in doing that with LBB5.80).
The short answer is you can do that under High Guard, and kinda under Book2. Book2 basic drive increment is 200 tons of performance per whole letter.
You CAN do a 150 ton starship under LBB2 (for example) ... it's just that you wind up designing it as a 150 ton custom hull and use the 200 ton row for drive letter to number conversions. This is inherently "inefficient" to do, but it does allow some wacky things like needing only 30 tons of fuel for a jump-2 out of a Jump-B drive in a 150 ton hull (instead of the expected 40 tons for a 200 ton starship).
It's one of the reasons why I never cared for LBB2 starship design myself, since it's all organized around Table To Results ... rather than the LBB5 system of Table To Formula For Results which makes hull sizes in increments besides multiples of 10 or 100 tons more useful to contemplate (as I seem to keep doing).
If you're familiar with my posts in The Fleet forum over the past ~4 months or so, you'll have noticed that I keep using 194 ton hulls for things ... not because I set out to use that hull size to the exclusion of all else (as a personal design quirk), but becuase the mathematics of starship design keep gravitating towards that 194 ton hull size in a kind of Strange Attractor sort of way as being the most efficient hull size given what else needs to be stuffed inside of it (drives, fuel, bridge, computer, turret, staterooms, cargo, etc.). Somehow, I keep landing on 194 tons as being a tremendous "sweet spot" for LBB5.80 low end civilian starship designs for a wide variety of reasons.
And oddly enough, I'm working on yet another starship design that is once again modeling out as being 194 tons of hull is the optimal hull size (because 18% of 194 is 34.92 tons worth of drives, meaning you only need 1 engineer per 35 tons of drives if you're going to have any engineers at all). This is "true" not because I want it to be ... but because that's simply what happens with a Jump-3 (4%), Maneuver-2 (5%), Power Plant-3 (9% @ TL=12) starship hull fraction under LBB5.80, and the number to "reach but not go over" is 35 tons of drives due to crew requirements (per LBB2 for starships 1000 tons and under). However, in this case, I might need to scale up to the full 200 tons anyway (and take the hit of needing to provide another stateroom for an extra engineer plus a medic), which you will notice means needing to add 8 tons more to the fixed displacement inside the ship while adding only 6 tons to the hull displacement ... meaning that going from 194 tons to 200 tons of hull size winds up perversely reducing the amount of cargo space available (because the drives and fuel tankage needs to go up too) which really puts the squeeze play on balancing the competing needs of everything that has to go into a starship while still making it all fit. It's just kind of perverse that above 194 tons you have to "overshoot" 200 tons by a good bit in order realize the same amount of cargo capacity (absolute tonnage) that you can achieve at 194 tons (or less, depending on specifics).
LBB2 doesn't give you that kind of mathematical granularity, since everything is predefined (and spoon fed) to you as a starship designer. I've always looked at the LBB2 system as a fine example of premature optimization ... because LBB2 "works" as a set of starship design rules, but it also railroads you into using extremely specific hull sizes for EVERYTHING and penalizes you for deviating from that course of design action.