Boost Gauge sits at -29 and constantly fluctuates outside of game

Hi,

I've wired up a digital boost gauge using this guide:
https://github.com/SHWotever/SimHub/wiki/Arduino-After-Market-Boost-Gauge

It actually works in-game, but it sits at -29. I've tried calibrating and adjusting but haven't had any luck.

It always seems to bounce / fluctuate between a few different digits 

If I don't have 'use full boost gauge range' ticked, it will constantly fluctuate between -29 and a few numbers. If I tick use full boost, it sits at -29. even after adjusting.

any ideas? 

Attached video: 

https://imgur.com/a/UXLbzxu

Thanks