My two both get a tablespoon twice a day. Farra being a 17 hand clydes has a heaped one, and Chancer who is just over 15 hands cob - about 500 k gets a level tablespoon.
If you are feeding for SI, I have also found feeding linseed and clivers helps as they both promote healthy skin. The clivers is a general cleansing good tonic and is also anti-inflammatory and I have noticed an improvement in their rubbing since feeding it in addition to the BY and linseed.
BY is not really for for flies but is used for midges who hate the stuff, or the vit B to be precise. For flies try garlic. I don't feed garlic as this can make things worse for SI suffers.