Harrisons do not use AMO as a reference for string length. My 66" HHS uses a 59" string. My 64" HHS used a 58 7/8" string.
Leo (Rest his soul) would traditionally send three strings, whichever one fit best became the template and you returned it with the other two. He would then build however many strings you wanted based on the template string.
If you don't have a string to use as a template, you will have to get creative with some paracord and a bowstringer to determine a starting point, or find a stringmaker who is willing to send you a few strings to try out.
You are correct that FF continuous loop strings are preferred for Harrisons, and brace height is VERY finicky on these bows. A few twists are not a big deal for final tuning. It was when endless loops were twisted like Flemish style strings that Leo would pop a gasket.