I messed around a bit and got very weird results, after a while I think I figured it out. There's some legacy shenanigans going on with the depth values, so you need to explicitly read them from the layers. This version of the code seems to work as intended:
z = -64*(layer_get_depth(layer) div 100)
But you also need to do two more things:
- CONTROL currently changes its depth value in the create event, cut this line and paste it in the Room Start event (otherwise it gets an extreme depth value which in turn messes up where the player spawns on the Z axis)
- Likewise, all of the "floor" objects sets their depth to 0 in the create event, remove this entirely.