Mastering the scales and arpeggios, a complete and practical system for studying the scales and arpeggios from the most elementary steps to the highest attainable degree of velocity and artistic perfection – James Francis Cooke
…in the public domain, the book is available for free on Google Books.
There are sections within discussing 208 bpm as a target, and while it’s aimed at piano players, I’m finding some of the sections to be gems with regard to approaches to speed development.
Learned of the book, wia Josh Wright Piano, here: