yes sheen would boost it but if you tried it did you notice if the 150% from his ability do anything to it? Or did it calculate base off of your damage x amount of times?

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.

Put simply, Tiamat is not incredibly powerful on Renekton unless you stack them. And even then there are better alternatives - doesn't have enough range to do much besides help in farming.

thank you very much for your analysis and help. Regardless of the slightly less damage, I feel in theory at least that it will give you a higher output damage 1 v 2 melee champs then another item then say bloodthirster. The unique nature of that ability i think has a lot of potential with tiamat.