"Conversely, a pitcher with 180 IP at 110% probably needs a spot starter to fill in a few times furing the year, but at 115% could make every start for about 6 innings."

With the 115% rule, you can use 4 guys with 180IP as a 4 man rotation. This shouldn't happen.

180 / 5.66 IP = 32 starts = 5 man rotation = Real life

180 x 1.15 / 5.66 IP = 37 starts = 4 man rotation + 12 starts by a 5th starter

180 x 1.1 / 5.66 = 35 starts + 20 starts by a 5th starter.

With a 20 team league, every team has enough depth that 1.15 isn't needed.