The amount of right thrust used should be propotional to
A) the amount of torque which is anticapated will be used for accelerations, and
B) the pilots ability to counter that torque with right rudder.

Full scale single engined warbirds didn't generally have right thrust (altho some have a fin offset). R/C modellers use it to make it easier to fly, especially when taking off and/ or accelerating from a slow speed with high power.

I generally remove about 1 degree of right thrust from my Top Flite and Hangar 9 warbirdss. I'm thinking that it brings it down to about 2 deg.
Makes it slightly more scale looking, and I am able to fly it more like a real warbird.

As for downtrust: WAY too many variables, including (but no limited to) wing incidence, tail incidence, wing location (hi wing, low wing, or mid wing) and engine location (again: high mounted or mid level) to give generic answers.