GentlemanJonI found a bug with smaller resolutions when looking at this so I'll upload a fixed version soon.
The size of the map is scaled, so if your resolution is 1280*720 and your co-caster is running 1920*1080, then the proportion of width and height is 1.5, so if you have the map horizontal position at 800 then multiplying it by the proportion of width and height should give you 1200 and it should appear in the same place on your co-caster's screen. Using a different aspect ratio obviously means this is harder, it's not a use case I looked at during design.
The issue isn't for offset from the top left, specifically. The issue lies in if you want it to be affixed to the bottom right. Let's say, I have a 1600x900 monitor (which I do) and I want the minimap size to be 2 (doubled) and affixed to the bottom right. I have the horizontal_pos set to 1300 and vertical_pos set to 542. For 1600x900, this looks fantastic. However, as soon as you want to give this to another user (1280x720 or 1920x1080), you need to recalculate everything, remembering the size and offsets from max. It's a pain and while you could do it manually with trial and error for smaller resolutions, it's near impossible for larger ones.
Raw pixel offsets with anchors is honestly the best solution. Every decent GUI framework out there, even WinForms, uses this method.
GentlemanJonI'm aware of this and from memory it can be solved in first person, but then causes unstable display in third person. However my understanding has moved on from then so there might be a solution now. I'll look into it.
I think it would be totally acceptable to just disable it in third person as well if you can't find a proper workaround.