Usually a 100 pt scale 100 is the MQL. Sometimes the MQL is 60 or 70 and there's a decay process.
I'm more concerned about buying signal behaviors than actual score though. It's too easy to get to 800+ through website visits alone. Are they a good fit? And did they hit the 2-3 key behaviors for your audience?
That being said, can you analyze the leads that were SAL or went to Opps and see where there score was when they went over to Sales? Is there a threshold that would roughly work?
I agree that the decay process needs to be well thought out. I've seen inactivity at -10 every 30 days or so. Unsubscribes can be -2 to -10....depends how much you believe that's a problem.
If your leads have ridiculous scores, then take that group over say 200 points and decay them or reset their score =50.