I guess it's a matter of interpretation. To me any ship with used MSP is in the "low maintenance" category and to be directed to a spaceport at their earliest convenience.
But I'm open to differing suggestions. What would be a sensible threshold?
70-80% of the ships in my list are survey ships and in my case they're the least of my concern as they have the 'if supply below 20%' standing order.
I'm guessing for each person the threshold is different as we each play the game in our own way, the suggestion I have would be to allow the user to select the threshold for which the warning should show. And an extra suggestion I would have would be to filter out ship classes.
As an example, if I'd be able to apply my own filter and threshold I would probably go somewhere at 50% and exclude my survey ships so I can monitor my just my warships and my space stations.
Don't get me wrong, the list is useful as is, thanks to it I just realized I forgot to re-instate a supply line to a space station after a refit, however, in my case, it took some time for me to notice this due to the list being mostly survey ships.