Eldo fork rebuild - shims

[BSABEN]

I am rebuilding my drum brake Eldo forks. There are shims above the upper slider bushings. I can't figure out what measurement these shims are intended to equal. I assume they are to even the overall length but what is to be measured?

I want to have this shimmed correctly but i have no idea what to measure / shim for!

[guzzisteve]

I think that is shimmed to keep travel of the bush to a minimum when tube goes through, less wear.