If you want to scale to an exact distance, one way would be to simply duplicate (shft d) the edge or two points from your model that represent the distance you want to change. § to extract selection. Take that new object and use the dimension field to add the math for example -.1 to take away 1 cm. If you then copy the scale values that result and apply those to the original object you will get the exact distance applied back to those original points.

Here is a video. Later in the video I describe the relationship between the dimensions and the scale values. Just make sure all your scale values are at 1 (zeroed out) before you start.