Sure, I've chosen the values arbitrarily at this point, because I wasn't sure what optimal values would be.
Why do you choose 20, just out of curiosity? I chose 8 at first, but that seemed to really crank the difficulty, then 12... we can move to 20 and see how that plays out.
You should find the difficulty which lowers the traffic to a minimum, whilst also minimising stales and maximizing hashrate (obvioulsy).
So pick a value to start with, then vary that (up and down) to minimise GW/m + shares/m.
The whole point of this Vardiff, is afterall, to minimise traffic.
Have a play, build a table, see what results you get.