Peter, what length stem do you have on your Sherpa? I am 5 foot 3 with 77cm inside leg, and Thorn recommended me 500L. I decided it was too long and after much deliberation, swapped for 475M (they were out of 500s for months in advance). I think it's ok with 100mm 15 degree stem, it's something for me to continually ruminate on! The Thorns are different than, say, a Dawes, because the front fork slopes back a lot (sure there's a technical term for that!) so a Thorn with a 54cm top tube may not feel the same as a Dawes.