Honestly, if you've been gone for a long time, it might be best for you to just reroll a new account with a 100% Eidolon.Never. Eidolon Orbs are for Eidolons that you missed (such as Tiamat for Water) or for Kaisers.For 120% Eidolons, the 5% goes to base. They're always 100% base + 5% per Star + 4% per same element Eidolon equipped (will be buffed to 8% in December).
However, please notice that Managarm and Anubis work differently. They're 80% base + 4% per same element Eidolon equipped + 1% per Star per same element Eidolon equipped. So a 4-Star Anubis is still 80% if you have no Dark Eidolon equipped.Assault is 0% base for (), 3% base for (+), 6% base for (++). 0.5% per skill level, so 10% at slvl20 and 15% at slvl30.
So a slvl20 "Large" Assault is 16%.