Many thanks. Not quite a 100 I don't think - perhaps 100km.
I've seen Shasta from the summit of Lassen Peak. That's 100 miles to the south and from Mt McLaughlin in Oregon - that's perhaps 100 miles too.
I swear I've also seen it from the summit of Theilson (~150 miles) and South Sister (>200 miles). I've also seen Mt Baker from the summit of Adams (~175 miles) - and there I've got the photo to prove it - in cold and, therefore, clear conditions.