It is not theoretical at all: I have set profiles for my mini without mag as low as 25. With a Safe Speed of 40% the top speed is still below safe speed.
I was thinking at one more sophisticated solution. Introducing a Minimum Viable Speed (MVS): this value is between 0-63 and is the lowest speed that allows a car to move around the track (clearing tight corners and slopes). Leave the Safe Speed as it is (% of the top speed of the current profile). However, if for a given profile the Safe Speed value is below the MVS, then the MVS value is used as Safe Speed.
The idea is that the MVS allows the car to crawl around the track. Below this speed the car would get stuck (which is not acceptable).
Safe Speed is not for crawling: it is a reduced speed compared to the top speed but still allows the car to move around fairly quickly to clear the pit or at the end of the race.
But I realise that is too sophisticated for Martin: so no way that is going to be implemented. ;-)
One last comment: Martin given that you are going for Safe Speed as a % of 63, then i would suggest to drop the percentage and have Safe Speed as a value between 0 and 63. Less risks of confusing your users.