I attended a contest under new rules that mandate the use of Sig Shoestring or Buster kits. There was some talk of the boredom of only two designs and the fact that a Buster won. I did some study of the 1/8 scaling of Goodyear racers. I noted that one modeler rather casually mentioned the intentional use of the -5% tolerance.
The following numbers are listed in this order: Model, -5%, true size, +5% and Sig kit size.
Buster 21.5", 22.7", 23.8", 24" (conclusion, the Sig kit is oversize illegal)
Shoestring 27.1", 28.5", 29.9", 28" (conclusion, the Sig kit is within tolerance on the small side)
It looks like if I want really small, a 95% Cassutt could be built with a 19.5" span. But i'm not really sure smaller is better in this case.