I don't think you're grading scale works to be honest mate
Koeman came in when we were a bottom half side. If he wins the title next season, but then leaves us when we're 17th, he'll have dropped further and sharper then Martinez or Walker ever did
However, he will have also won the League, so what means more? How high you can get the club or how low you take it?
Under your grading scale you could argue that this would make Koeman worse than Martinez, because he took a club from 1st to 17th, but we all know that rationally Koeman's legacy would be that he was our best manager of the PL Era at that stage because he would have brought home title number 10
You have to look at everything that happened in a managers reign. Martinez getting fifth whilst having us play some good togger puts him ahead of Walker, who had us in the bottom half playing dross from start to finish
Fair is fair at the end of the day. I consider Martinez's managerial run to be a failure, but a failure isn't as bad as a catastrophe, and that's the only way you could describe Walker's run. Literally the only "positive" memory of his reign was beating Wimbledon 3-2, he had nothing else to his name. Martinez had the Oviedo goal at Old Traffic, the Arsenal game at home, Wolfsburg away etc
It's not even close, Martinez > Walker
Sod it, Walker is in the running to be our worst manager ever, he was utter mince with no redeeming qualities