Soooo, I just did some numbercrnching with the spreadsheet, and I think I've worked it out...
Basically, the sword spells add a multiplier (1.1 and 1.2 for the first and third level spells), then SoM and it's epic counterparts add another multiplier. In the case of two SoM skills, it adds a multiplier for each skill (2 total). It looks like the multipliers are as follows:
Level - Normal / Epic
E - 1.05(*) / 1.02
D - 1.08 / 1.05
C - 1.12 / 1.08
B - 1.15 / 1.10
A - 1.18 / 1.12
S - 1.20 / 1.15
SS - 1.30 / 1.20
It also includes a multiplier for the elemental affinity corresponding to the spell. I've used the following values for these...
Affinity Level - Multiplier
E - 0
D - 0.5
C - 1
B - 1.1
A - 1.2
S - 1.5
...which I got from the results with no skills. This gives a final calculation of:
Damage = Base * Spell * Affinity * Skill (* Skill 2)
For example, Kyle with both Sword of Magic and Zen Sword equipped and at SS would do 1 * 1.2 * 1.5 * 1.3 * 1.2 for about 280% (2.808) of his normal damage, pretty much exactly as quoted in the first post and elsewhere.
I think that's about it, at least for the data from the spreadsheet... the numbers all work out, anyways.
If anyone wants I can email them the edited spreadsheet, but it doesn't use any numbers that weren't already there.
...Now if only my playstation was working so I could confirm all that
(of course if it was working I wouldn't have done this in the first place, but meh, details...
(*) A value was excluded from this dataset as it would've pushed the multiplier up to ~1.07 if it was accurate, which sounds too close to D's 1.08.