AJ Delange, the brewing chemist extraordinaire, says to never add alkalinity.
Using only distilled water, and 210 trials, only 4 combinations of grains fell below mash pH range. SRM is not a reliable estimator of mash pH.
So add in ANY amount of alkalinity for your water, and you'll be probably be fine without any additional alkalinity. The only way to know for sure is to measure your mash pH after dough-in. Delange also says that the pH strips commonly available are biased to be -0.3 pH from what a pH meter would read, so a reading of 4.9 would really be 5.2.