@Silent. I think 0.65 is a good place to start. You could then watch between waterings to see if either show signs of distress just before the next watering. Based on that you can adjust one way or the other. I'm doing that now with my shrub zone that has a decent spread of coefficients.
It would be nice to separate those trees to a different zone if you ever get the chance. Do the trees at least have more emitter heads to them? They're roots are deeper and spread more widely, so I would think that they'd eventually suffer.