Re-scaling question (Stargrunt)

I'm planning on running some games using Stargrunt II and a lot of my
W40K, Grenadier Dark Future, and a few Champions miniatures.  I already
have them, and I've accumulated a lot of buildings, vehicles, and
terrain in the same scale.

The problem is that scale is about 28mm ('O' scale for train people),
not the 20-25mm that Stargrunt II is intended for.  That might not seem
like a big difference, but I think the 'official' Stargrunt minis are
closer to 1/72 scale than 1/54 (25mm), and most of my minis are 1/43.

The upshot of all this is that with such large miniatures, a range band
of 6" for Green troops is going to result in troops that are almost
standing next to each other who can't hit each other.

Does anyone have any suggestions on re-scaling the movement rates and
range bands to account for this?  I'm thinking maybe multiplying
everything by about 1.5 would fix it, but I was wondering whether anyone
else had tried anything like this and could lend me the benefit of their
experience.