Paul,

it might be 80% (i could check .. in fact I have checked for you below) but it doesn't matter if 80% of all runners run within 21 days.

Total horse runs in my DB : 195870
With 21 days last start or less : 151160
77%

Total winners : 20344
with 21 days last start or less : 12462
61%

This highlights something very interesting but needs the right questions asked about it. The above is very useful for a ratings system. Are you creating a rating system ? If so you just learnt something very useful. If not then you just learnt nothing useful.

But overall your asking the wrong questions if you trying to make a profit on horse racing.