All pruning cuts should made back to branch line and bud in accordance with BS3998 where possible. Topping and lopping procedures should be avoided.
Tree must be identified by species and overall inspection of the tree must be made to evaluate its current health condition. Decisions can then be made as to what pruning procedure would best suit the tree and at the appropriate time of year. It may not be necessary to reduce the whole crown structure, but consider balancing the crown shape and form.

We would recommend when removing more than 30% of the foliage, that you consider dividing the job into 2 sessions, around 12 months apart to minimise sprouting and starch removal from the tree. To reduce the size of a tree with drop-crotch cuts, we shorten the branches that extend beyond surrounding branches. This maintains the approximate original shape of the tree. The tree is simply made smaller.

The longest portion of the main branches will be cut back to an existing, smaller lateral branch that is large enough to assume the role of the branch. This is normally a third to half the diameter of the removed branch.

Excessive sprouting accompanied by die back or decay often occurs if you cut back to a branch that is too small. It is unreasonable to expect more than about a 15-20% reduction in size of canopy from a properly executed crown reduction.

This is a time consuming technique and is more an art than a science. It requires substantial talent to perform this operation. Of course this is a temporary measure because the tree will quickly revert to its natural size.

Best practice would be done by someone experienced in tree climbing and knowledge of when to prune certain trees. A mobile elevated platform or for someone who again is experienced should be done by rope and harness. Never use a ladder, I have to many Arborist friends who have serious neck breaking injuries from using a ladder.

20%-30%

Normally 20% is good amount to keep the tree in good health. Where possible the use of a MEWP is normally the safest option.

Crown reduction not a Pollarded

Anywhere between 20% to 30%.

No more than 30% crown reduction

When reducing the entire crown of a tree there are a few factors that have to be considered.
Firstly the species of tree can effect how much you are able to take off without killing the tree or putting it under unnecessary stress.
Some trees react better then others and can handle harder reductions, the amount is usually referred to in percentages.
50% would be considered the maximum amount and would only be advised on dangerous trees.
10% would be the lightest and depending on the size of the tree roughly 1m taken off all round.
A good arborist will cut to growth points on each branch of the tree, this means that the process of sealing off the wound made from the chainsaw minimises the energy and nutrients the tree has to use to heal itself and insures the growth point left becomes the appical bud of that branch.
This means the tree will continue to grow in a unified and balanced way over the coming years.
Most deciduous trees are best reduced when the buds are dormant so winter time is best but many trees are also reduced through summer and still survive.
Hope this helps