I've looked into this a fair bit myself, actually. You have a fair number of options for Level 1 Towers. As a tidbit, you need at least 81 max magic to cast FtHoF twice (at least 307 Towers), selling down to 23 max magic (21–22 Towers). The first "sell in 100s" number of Towers is 320–321 (82 max magic), selling down to 20–21 (23 max magic).

In terms of other options, this isn't an extensive list by any means, but here are some "sell in 100s" setups that I've used myself:

• 421 Towers, selling down to 21 (0 magic remaining)
  
• 431 Towers, selling down to 31 (1 magic remaining)
  
• 521 Towers, selling down to 21 (0 magic remaining)
  
• 531 Towers, selling down to 31 (2 magic remaining)

They aren't necesserily optimal, though, as they leave you with minimal magic left over (and so slower magic regen), they're just what I've used.
In terms of having maximum magic left over, and having Towers less than 630, I think the most optimal closest to that looks like 557 Towers (103 max magic) and selling down to 57 Towers (32 max magic) which will leave you with 6 magic left over.