4th job Single Target DPS

Uh... one formula I've seen that works reasonably:
Charge #1: calculate normally
Charge #2 (always Lightning when 2 charges are present): divide by 10 the result
Add the 2 to get output total.

Eg. Holy (weak) + Lightning (neutral) 150% + 125%/10 = 162.5%
Holy (weak) + Lightning (neutral) 150%*150% + 125%/10 = 237.5%

Further ingame testing has proven this accurate everywhere I've tested it. I'm fairly confident this is a reasonable way to calculate dual charge damage.