Spreadsheet calculates 42562 avg expiacion at r15 vs 32545 at r0, with 3435 of that difference coming from the +18 base dmg difference alone. This leaves ~20% damage increase from the +15% r15 bonus which is about right (49.5% at r15 total bonus to expiacion vs 30% at r0).
(42562 - 32545)/32545 = ~.308 total difference between r0 and r15
3435 is the difference between r0 exp damage and r15 exp damage BEFORE mythic/r15 bonuses but AFTER applying +18 base dmg according to the spreadsheet. (I can't easily see how this number is so high though: I get at best a ~5% increase from +18 dmg)
3435/32545 = ~.105
∴ the rest of the r15 aug (IGNORING base dmg +18) accounts for ~.203 of the difference ADDITIVELY.
Now if you truly don't understand the semantic difference between additive multiplier arithmetic and multiplicative multiplier arithmetic then I can explain in more detail, or you can take my word for it that the ADDITIVE difference between 30k and 34.5k is ~.195 when the BASE is 23k (as in your example).
Edit: To make it simpler (and make this post more informative), on the spreadsheet:
(1) r0 Expiacion damage BEFORE multiplying by 1.3 = 25034.92
(2) r0 Expiacion damage AFTER multiplying by 1.3 = 32545
(3) r15 Exp dmg BEFORE multiplying by 1.495 = 28469.88
(4) r15 Exp dmg AFTER multiplying by 1.495 = 42562
Subtracting (3) from (1) gives the amount contributed by the +18 base dmg before mythic/r15 multipliers. (I don't see how this number is so high, although Simon's approximation is way off base).
Here is my approximation:
Damage = Base Damage *pDIF
Base Damage ~= (Weapon Base Damage + fSTR + WSC)
ratio of r15 to r0 ignoring mythic/ws bonuses = (basedmg + 18 + fSTR + WSC)/(basedmg + fSTR + WSC)
Where STR=343 DEX=330 INT=223, target's VIT=200.
=>
((147 +18 + (343-200+6)/4 + .3*343+.3*223 +.2*330))/((147 + (343-200+6)/4 + .3*343+.3*223 +.2*330))= ~1.042
Compared to the spreadsheet numbers:
(r15 Exp dmg BEFORE multiplying by 1.495)/(r0 Expiacion damage BEFORE multiplying by 1.3) = ~1.137
So either the spreadsheet models base damage contribution to weapon skills incorrectly or I am clearly missing something.