Scale problem on the drawing

I have a general view on the drawing, scale is set to custom.

in the relations of the model I calculate the approx. scale and set it to a param. SC1. then i set that param to the scale of the view. The model is used as a template - there are different sizes of the actual product, but the drawings are mostly the same.

The problem is that when i work on the model further and i want to manually fine-tune the scale, it sometimes jumps back to the value, preset by a param, when i regen the drawing. 

This happens only on some drawings, on most it stays set to the value i manually input (when i override the set value).

it doesn't help if i delete the equations and parameters. when i paste them again into the model relations, it happens again.

anyone has any idea?