As a first approximation - a ratio of 60% dappled shade to 40% light reaching the river is not a bad starting point. There's nothing wrong with creating some scattered clear patches. If you have the resources, rotational coppicing would be a good option. This way the trees are encouraged to grow at a variety of heights instead of all being one uniform blanket.
Chunky bits of wood arising from such coppicing would also enhance things by being left (or deliberately fixed) in the channel.
Definitely do not remove large tracts of low "fringeing" or overhanging branch cover. People might lose flies on them, but there are seldom any fish to cast to in the absence of this low-level cover. Coppicing is good in this respect as the low bushy regrowth is excellent low-level cover.
One or two "tunneled" sections of deep shade will become more important with climate change - to act as refuges from scorching hot weather and low water conditions. Such tunnels also deter cormorants, as they prefer a clear vertical line of sight for takeoff. Similarly, introducing brash/bushy tree tops (either by wiring and pinning in place or by partly cutting through trunks and hingeing the tree top into the water) make excellent anti-cormorant and general anti-predator refuges for juvenile and adult trout. Hingeing trees in this way will also open up overgrown canopies.
Hope these suggestions give you some good ideas. The WTT are active all over the UK and can be approached for Advisory Visits (free apart from travel expenses) via the website:
Wild Trout Trust
PG