Flying through the ash
These test flies are just the beginning of the things that may be necessary to do. Even when they establish certain threshold of ash concentration, below which it is "not dangerous" to fly, that is not enough, nor does it mean very much. Below critical ash density can also cause damage over time. Two most important parameters are probably accumulation of the dust inside engines and other critical surfaces, and accumulation of the microscopic dust damage - and not simply local dust density. And to be able to predict them in advance, during the trajectory planning stage, it will be necessary to have some very sophisticated monitoring system. If this does not stop very soon, dust concentrations will become almost unpredictable, possibly abruptly changing depending on the place, time, altitude, moisture, history of the winds at various levels and so on. Detailed remote measurement and on board sensors may be the only way. I am not sure if remote monitoring system with necessary high resolution for measuring local densities at various levels even exists now. And even if it does, it will be cat and mouse game. Finding the current route with the smallest ratio of accumulated dust / cost of the trajectory. And constantly hoping that accumulated damage to the system has not become greater then its ability to withstand some unexpected stress that may be required. It's not going to be easy. And it is certainly going to be more expensive. So lets hope that it will stop very soon.