You make a fraction. You have two scales, the original scale and the desired scale. So you need to make a fraction- one scale divided by the other. The problem is deciding which is the numerator, which is the denominator. It is easier to just use common sense here rather than rely on a mathematical formula. If you are trying to reduce scale, to a smaller model, the value of the fraction should be less than 1.0. If you are scaling up, trying to make it bigger, you should end up with a fraction with a value larger than 1.0. You can tell just by looking at your fraction which is the case. If you want to scale up, the larger number should be on top, the smaller on the bottom. To scale down, the smaller number should be on top, the larger on the bottom.
The way you say it, it sounds like you are scaling down from some larger scale (smaller number). Let's say 1:350. Scaling down, you want a number less than 1, so if we put 350 on top, 700 on bottom (350/700) that is less than 1 so that is the way to do it). Answer in this case is trivial, result is 0.5- cut all dimensions in half.