Hello!
I need some clarification please.
By saying "number of runners", do you mean the total number of runners in the race (i.e. minimum 8 runners) or the number of runners that participate in the Dutching?
Because you can EITHER specify the field percentage you want to cover OR the number of runners to back on, not both at once, as cases when both values match your variables will be extremely rare.
Also, is 80% the maximum or minimum value?