Here's the simple way to resize blocks:
Start with the size you WANT divided by the size you HAVE.
In your example, you WANT a 12" block, but you HAVE an 8" block.
Take 12" (WANT) divided by 8'' (HAVE) and you get 1.5 That would be 150% to increase on a copier.
If you WANT an 8" block, but you HAVE a 12" block, you do the same thing.
Take the 8" WANT and divide by the 12" HAVE to get .6667. On a copier, you would select reduce to 67% to get your new size.