You can't abandon story missions.

To tune it you just add up the percentages listed on parts from the the tuning store. Can't be under or over the listed ammiunt.

Might wanna try checking out the YouTube website for some Tips & Tricks .

35 isn't between 15 and 20.

Note pay out for that portion of the job is for the cheapest parts to make min %, so can be easy to lose a lot of money on the job. Just need some basic maths to add up % and $ per part, so don't be using a 3000cr gearbox for a 1% gain when most parts are 170-180cr* per 1%. Doesn't need to be balanced or make sense, if just one sparkplug out of 8 gets you over min% then do it.
*depending on "discount skill"

Yeah, those job order missions threw me at first. For example, I had a job that wanted a 10% performance boost. I used some parts that added up to 11% and the game didn't accept it. I had thought that going slightly over would not be a problem. But, it is. You can NOT go over and ofc, need to make the minimum. Steam user @astrofelix59 has nailed it... exactly like they said.

ohhhhh ya can't go OVER. thats where I messed up. Thanks everyone.