You are mixing MB (10^6) and MiB (2^20) units. Your "8MB" PE is actually 8388608 bytes, and 15 of those is 125829120 bytes. Then you have to add the 192KiB (196608 bytes) for the LVM PV header, so the total space needed is 126025728 bytes.
Your partitioning tool (Which one?) is probably using power-of-10 sizes for the partitions and then rounding that to the next MiB, so by that calculation you would to ask for 126MB or 127MB as the partition size.