Good luck with that....
Thankfully the car will run just fine without anybody watching the gauges.
Regarding C-to-F temperature conversion: there must be several examples of this already out there around here. You'd multiply by 9, divide by 50 (because you were already dividing by 10), and add 32. 9dec is 9hex, 50dec is 32hex, and subtracting 32 more when you are already subtracting 40 is -72dec = -48hex = +FFB8hex in two's complement, so that would make the MTH 00090032FFB8. If this blows up your scangauge or your engine, don't blame me.
And the next time somebody asks, it's your turn to explain it.