Douglas Harper's Etymology Dictionary
Latin, literally "for so much; to such an extent;" see pro- + tantamount.
Wiktionary
adv. (context linguistics English) Only to that extent.
Usage examples of "pro tanto".
As between individuals it might adopt the mutual insurance principle pro tanto, and divide damages when both were in fault, as in the rusticum judicium of the admiralty, or it might throw all loss upon the actor irrespective of fault.
Their vain fears only substitute other and worse evils for those which they are idly apprehensive of: while every restraint on the freedom of conduct of any of their human fellow-creatures (otherwise than by making them responsible for any evil actually caused by it), dries up pro tanto the principal fountain of human happiness, and leaves the species less rich, to an inappreciable degree, in all that makes life valuable to the individual human being.