There is a more better way to fix this. Instead of offsetting the standpipe from the shower drain to the trap. move the trap to a point directly under the shower drain.
Buy a new trap and a 45°. Insert a length of pipe into the drain body with the trap attached at the bottom roughly the same height as the horizontal trap arm and adjust it til it points at a 45 degrees to the trap arm. Cut the trap arm where that 45 lines up. then cut a piece to go from your 45 to the trap. The trap has a bit of "swing" to it, so you can adjust that to the vertical when you get there.