flexible option with @#~µ²³ STEP assembly file

[edo777]

dear all,

I am stupidly stuck into something that most likely is a stupid thing, but I can not get out of the thing.

I am assemblying an XYZ stage and have to check for different envelope aspects.
So, I import the STEP from the manufacturer, and I assemble the very same stage along X, Y and Z.
I reassemble the moving stage with respect to the fixed part so that I can vary the position of the moving part by imposing a distance.

Then I tried to create a flexible assembly, by taking the distance quote.
So, first thing, I can not grab the assembly quote as a flexible option. And I get aaargh.
So, I create a parameter "position" and the I generate a relation between the parameter "position" and the assembly distance quote.
So, I can make the bloody thing flexible, but only once. To change the position of the stage, I have to remove the flexibility and reimpose the flexibility on the same parameter and re-introduce a new value..

What the @#~µ²³ am I doing wrong?
I provide you the stage, in case you have an idea...

thanks for any suggestion!
Edo