Legacy GM 2.5 Zooming Game, Get Image Scale for Zooming from Obj Position Relative to Player on Top View Map

I want to make a 2.5 game that zooms like Space Harrier, but want the ability to vary speed, stop, maybe to be bounced backwards. My plan is to place icons of the scaling objects on a top view map. I want to base the image scale on the distance the player is from an object (that is map_obstacle.y-map_player.y) on the invisible (unless testing) top view map, but what equation will get correct z scale from that distance info. The image should be drawn at the calculated size on the actual in game visible screen. If I know the z scale I think I can get depth and proper x, y coordinates for the game screen too by multiplying.
 
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