http://wiki.ffxiclopedia.org/wiki/Calculating_Blue_Magic_Damage
About a third the way down the page is a table stating D cap for spells, followed by:
"The D cap seems to be capped at 14~15 level after the skill can be used.
Since Jet Stream can be used at Lv38, the D is capped at Lv53 with Blue Magic skill 164~168."
From this, and assuming a combat skill increase of 6 per level (this bit will almost certainly vary) I made the assumption the D cap for current physical spells would be around 102, probably a bit higher.
However I am probably misinterpreting the equation, is this the D cap gained purely from skill rather than the final D value?
ie. is it
102 cap = floor(D(from skill)+fSTR+WSC) * Multiplier
or
Final D (no cap) = floor(102 cap+fSTR+WSC) * Multiplier
:S I am probably not making sense