I'd agree about short breaks Kenny [6 days is my cut off point from similar studies], as it is a good bet the horse pulled up very well from it's last start. The general 21 day rule is a good one overall though.
I had 5 runners for a system I'm looking at on paper running today and 3 of them passed all the rules except the 21 day rule and were 'freshened' horses from 25 to 28 days. 1 placed 3rd and the other 2 were unplaced [all 3 started favorite]. The 2 within the 21 day rule both won.

Have a look at the stats for 1st up runners after 60 days or more break [2nd. up too for that matter] for the meaning of the word 'dismal', yet I'm sure there are systems that revolve around those horses.