If these two rules had been applied to yesterday's racing:

1) A minimum gap of 4.5 kilos between top and bottom weight,
2) Bet the top half of the field,

then there were 24 races in which the winner was struck in 19 of them.

Total outlay of $142
Total return of $186 on Unitab.

When I get time I'll have a further look at it. However I'm sure that if the system had merit then someone would know about it by now.