Welcome back carjack!

Simply take the scale you have and multiply it by some factor to arrive at the scale you want. If downsizing then the factor is less than 1. If upsizing then it's greater than 1.

Okay, so with the basics down, it's my understanding that the current kits are 1/64th scale, where 1" = 64 real-life inches. This is also known as 25mm scale, or in railroad terms, "S" scale. Some might say the current kits are more 1/60th (classic D&D where 1" = 5'), or perhaps 1/58th (which is the true scale for 28mm figures). But let's not quibble over the starting point.

You wanna get to "HO" scale, which is 1/87, or 18.5mm.

So if you take 25 (mm) and multiply that by .74 you get 18.5 (mm) which is exactly what HO or 1/87 scale is. So print at 74% and that should do it.

Now, I say "should" because artistic license has been taken with these terrain elements. The best check for proper scale "fit" is to find a component that has a door and try that. Then compare that to one of your minis (assuming of course that you have one in the desired scale).

If the figure looks like it could pass through the door about right then you've got the right scaling. If you figure looks like it need to duck to pass through, then the terrain has been scaled down too far. If the doorway looks far too tall, then the terrain hasn't been scaled down far enough.

This might be hard to check if all you have are big mechs.

If you feel the current terrain is more fitting for 28mm then your scaling is 66% rather than 74%. But like I said, the best visual check is to compare a standard door to a standard human mini in your target scale, and see if it looks right.

Hope that helped.

When I scaled mine down I used that option, taking a standard infantry and fitting them to the doors.

I think for mechwarriors its scaled doen 30% +/-