Depends...if the string is measured correctly according to AMO specs, 58" will likely be short or at least on the high side of the brace (no room for adjustment). This is based on my currently owning at least one Sage and setting up several more (not just making the string, actually setting the bow up to shoot).
Brace height can vary depending on the individual, but there's a range to stay in so you don't damage the bow/stay within the warranty.
You are asking good questions, and doing your homework. Unfortunately opinions and experiences can vary.
Generally a custom string maker will guarantee their work, i.e. if I tell you a 58.5" string is the right string and you determine you need it longer or shorter, I'll replace it at no charge. Both endless and Flemish can be adjusted some with a few twists, but you don't want to add or remove an excessive amount.