Mana/5 and get them per level.
Even with full mana seals (flat or per level), that's less than two extra boomerang tosses. After five minutes, if you're past level four, the per level mana regen seals will give back more mana than you get added from the mana seals (flat or per level). And not only do they get even better with each level, but Chalice of Harmony makes them more effective.
I know from experience that with full mana regen per level seals and chalice of harmony you can spam Boomerang Blade and keep Ricochet up. They're way more useful than a little extra max mana.