Well, I don't know if there is any sort of "formula" for that specifically.
If you wanted to scale something up or down, you just have to make sure you scale everything exactly the same.
For example, say you want to scale something down to where it's only 20% the original size that is 1/5 the scale. If something was 60 inches in height, just divide by 5. If the width is 30 inches, also divide that by 5.
If you want to scale up instead, you just multiply. So if you want a 10" x 3" object to be scaled up by 300%, That just would be 10 times 3 and 3 times 3. So 30" x 9".