As far as the sliding is concerned it works precisely to the mm as long as the scale of the object is applied. In object mode press ctrl-A ->apply scale. Check object properties or n-panel under “item” if scale is 1.000 in all axis. Then in edit mode you can slide and move your edge loops, vertices etc in any direction with g->axis you want to move in->amount (check top left corner).
Scaling is a bit different as the scale value is a percentage value. To grow an edge loop on let’s say a cube in the middle by 50mm total in both x directions you have to select one edge and move it by 25 in X direction, then the opposite side by -25 in x direction. Now your whole loop is 50mm bigger in X.
Alternatively if you can use the shrink/fatten function with alt-s, which goes by the amount. Make sure to press S again to check even thickness before pressing enter. (top left corner to check).
