My target box's seem to scale well to the object size:

Code:
tgt_dist = vec_dist(camera.x,imp_unit.x);
   	
draw_line(vector(temp.x-32768/tgt_dist, temp.y-4096/tgt_dist, 0), NULL, 40);
draw_line(vector(temp.x-32768/tgt_dist, temp.y+4096/tgt_dist, 0), vector(255,255,255), 40);
draw_line(vector(temp.x+32768/tgt_dist, temp.y+4096/tgt_dist, 0), vector(255,255,255), 40);
draw_line(vector(temp.x+32768/tgt_dist, temp.y-4096/tgt_dist, 0), vector(255,255,255), 40);

If you want better accuracy then I think the object's collision box dimensions are held in the vectors my.min_x & my.max_x

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First person submarine combat simulator project
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