Since the I-55/69/269 Hernando interchange is situated at the southwest terminus of I-269, it would be simple to interpolate the exit numbers simply by calculating the mileage of each exit from the center of that terminating interchange eastward -- unless MSDOT elects to do something out of the ordinary and continue the I-69 aggregate mileage (which, IIRC, is not posted on the E-W "Tunica" segment shared with MS 304) instead. But I'm guessing that they'll simply start with zero at Hernando; TN mileposts their segment from the MS/TN state line.
Or one could simply wait a couple of months and see the finished product.