If you think of it in incremental terms or marginal increases in space vs marginal increases in barn tools required, then there’s no real reason to stop upgrading just on the numbers.
If you accept that a core holding of say 300 tools is needed for barn tools, and always allow that to be allocated for barn tools, then on an incremental basis you require only 3 more tools to get 75 more space.
There are other ways of looking at it mathematically but that’s one of the simplest which demonstrates in the long run it makes sense to keep increasing