I used to have a excel spreadsheet about model scales and how to convert, say 1/56 into 1/50 scale. This sheet told you how many % you had to enlarge or reduce the size of a given model to go from eg 1/56 to 1/50.
Can anyone help me out?
Cannot help you with the sheet, but the actual calculation to get the scaling factor should be dividing the source scale by the target scale, in this case 56/50, which comes out to 1.12, i.e. you'll have to print a model scaled to 1/56 at 112% to have it scale out to 1/50.
E.g. a 1000cm/10-metre long vehicle would measure out to 17.86cm in 1/56. Multiplied by 112%, this comes to 20cm, i.e. 1/50th of 1000cm.
In reverse, a 1/50th scale model printed at 89.3% should measure out to 1/56.
EDIT: Note that this assumes the scaling is done by the individual dimensions (L/W/H) of the model. If the scaling is by volume, then the calculation would be rather different.