I swear to you that if you do this work properly, you will one day have a river of flowing gold" - Zozimus (Alexandria 3rd century AD).

Appropriate motto. About the right number of characters. From the right time period. It is the instructions (or more of an admonition) on the correct way to construct the Philosophers's Stone. However it's probably wrong. But that's what I would have encoded on the Kryptos.