Arbutus: Pruning Arbutus Trees

We would like to prune our arbutus tree. Height of tree approx 50ft
Our plan is use the local tree people, however we have a few questions:
Is there a right time?
How severely can it be pruned? We are looking at removing approx 30%
Thanks

 

If you are hoping you can shrink it down significantly it probably won't appreciate that too much, plus what happens after it grows back? If it has some deadwood maybe cleaning that out will enable you to see through the tree rather than over it, if improving a view is the objective.