What is the typical composition of English Breakfast tea? "English Breakfast" tea usually combines blends from Assam, Ceylon, and Kenya.

Typically what proportions of these three teas are used?

Edit: I have been asked to edit to explain why the answers to the question "Is English Breakfast the same as Assam?" does not answer this question. The reason is that the answers to that other question do not say what proportions of which blends typically constitute English Breakfast.
Well, blending is not easy and to do it in order to have a consistent product it requires quite the accurate nose. There is a reason why blender (for whatever product) is a very specialized job requiring natural disposition and years of training. 

And of course, the final product composition from both big and small companies is a very well guarded secret.

Here are some suggestions, you should try it out and see what you like. 

Assamica's Indian version of English Breakfast

Steepster forum (Tea lovers) discussing their own blends