Here is how to transpose a viola part written in “C” clef for English horn.
Summary: To make an English horn produce a concert pitch, play the note that is 7 semitones higher. Often you want to simultaneously go down an octave — in which case play 5 semitones lower.
Viola plays in “C” clef. It has it’s notes written one semitone lower on the staff. Oh, and one octave. So “middle c” is on the center line of the staff.
Armed with those facts, here is my recipe for tranposing a viola part written in “C” clef on the fly for English horn. First, look at the note as if it were treble clef (realizing it’s actually an octave less one line lower than it appears) then move it down TWO spaces or lines, and add a sharp to the key signature.
E.g. Suppose it looks like a ‘b’ and there are no sharps or flats in the key signature. (In viola’s “C” clef that isn’t a ‘b’ — the middle line is really a ‘c’ concert.) EH plays in “F”, so we must transpose down. The clef already moved it down one line, so we go down TWO more from ‘b’ to arrive at ‘G’.
Let’s take another example: Suppose the note looks like ‘a’. To a violist who knows the “C” clef, they would say, no, that looks like a ‘b’ — and they are right. To play a concert ‘b’ on an English horn, go down three and add a sharp. So it’s time to play an “F SHARP”. Simple!
A related, helpful shortcut: The same number of fingers rule!
When transposing a “C” part in TREBLE clef for English horn: If the note looks as though you’d play it with 1/2/3 fingers of the left hand as written, simply finger 1/2/3 fingers on the right. E.g. ‘b’ is one finger down. So is F#.
EDIT: The first time I wrote this I got thoroughly confused between how to transpose from Treble Clef vs how to transpose from “C” Clef. I think it’s correct now.