Lean the dog and the ground you run. No one can advice you on this barring perhaps someone that works with you. All dogs are different ; the speed, stamina and agility of a dog are all important so you need to Taylor the runs to the individual dog.
As for chance, if the dog has had work and know the score. Aslong as it has a 10% chance then slip it as otherwise you'll never know if you could have had an extra in your bag