I'd check out
http://wiki.project1999.com/Game_Mec...Q_Magic_System. In general that page is the goto for precise mechanics ... although it's not so much the final say on anything, just one of the best places to look.
Perhaps someday someone will work out the formula, make it super clear and easy to understand, and then add it to
http://wiki.project1999.com/Mana#Mana_.28MANA.29 (or better yet, create a new "Mana" page on the wiki for it).
If anyone ever does that I'd be happy to add a few form inputs (for level, + int, and + mana totals) and a drop-down (for class), and then add a tiny bit of code so that when people filled all of them out the wiki told you your exact mana.
And actually, on that note, there's also the
Magelo system in the wiki, which also calculates mana ... but I've heard reports its calculation isn't perfect either. Again, someone just needs to wade through all this, figure it out, and then make what they figure out clear ... if we ever want this to be truly answered well.