Looking at the equation from the JP wiki, it seems to be missing a sigma sign to sum up all the weapons and eidolons. Perhaps that's the source of this confusion.
Attack power shown for soul ={1000+LvUP_Bonus*1+(Rank×40)*2+[sum across 10 weapons(Weapon_attack_power×specialized_weapon_bo nus_1.2)]+[sum across 6 eidolon(eidolon_ATK_power×same_element_bonus_1.1)]}×(1+master_bonus*3).
I'd keep the event weapons separated and level them up individually until you fill out your grid. That's what I plan to do with the eidolons. It wastes some XP but it'll give you a short-term boost. Once your grid is filled with SRs and one weapon is capped, then LB it. If that short-term boost is unnecessary, don't waste the XP leveling a weapon which will just be scrapped.