It calculated off the damage I hit for. Two hits for 111 each splashes for 55 twice. Splashes less if I deal less damage too.
Renekton's ability does a TOTAL of 150% spread out over two hits, each hit's damage is boosted by a flat amount as well. The description is pretty weird - I feel it should be
Hits twice for 75% of his attack damage + base damage
Hits three times for 75% of his attack damage + base damage.