Well, he can't use 2S with that obsolete voltage regulator - L78S05CV requires 8v input! THREE VOLTS OF DROPOUT! That's just awful; straight out of the dark ages. Even a normal 7805 will get you 5v from 2S LiPo! A 7805 would work just fine here, as would a 1117-series regulator; 1117's are my go-to regulators, preferably the ZLDO ones, though every manufacturer has their own near identical take on the 1117)
Not sure why OP wants to use such a lousy regulator; I advise against it.
With anything that has less than 2.4v of dropout at 310mA as regulator, a 2S configuration is strictly better than a 3S configuration - the extra voltage is entirely wasted as heat, resulting in increased heatsinking requirements on the regulator for absolutely no increase in battery life. (ofc, with a switching regulator, it's a different story)