this lay approach will probably be OK;
these are the results for last years using 2 factors
1. DS>=100-1
2.2 starts or more

across every horse race covered by tatts bet since dec 2010
Values
starters # wins wsr wprofit wpot # plc psr plprofit plcPOT
172809 8340 5% -48683 -28% 28361 16% -44687 -26%

you would have to allow a bit more for the improved SP on BF - but there is plenty to play with at -28%

cheers
aussie