Dormant oil

I’ve been struggling with my two little dogwood trees for the past 3 years now. They suffered the first year I planted them from a terrible drought. Even watering them daily wasn’t enough to keep up. They weakened and I almost lost them. With a lot of pampering they started to come back. But last spring, they didn’t look so good again. On the advice of a nurseryman, I pruned the heck out them. Almost back to the trunk on one of them. I was doubtful, but it seemed to do the trick. They came back strong. But with all their suffering, they developed a case of scale which will eventually kill them if I don’t treat it. I’ve been told that spraying them with dormant oil in the early spring will kill the scale. But I’ve had varying advice on when to do it and, in fact, one nusery said that if I don’t do it at exactly the right time, the dormant oil will kill them. Conflicting advice makes it a little difficult to take the plunge. Question. Has anyone else sprayed ornamental trees with dormant oil with success? When is the best time? What conditions worked for you?