You're looking at it from a Skill vs Skill perspective, assuming LS falls on CT constantly every 50s. The falling of CT rotates between CT and FT, therefore you'd need to calculate the average PPS of both of the skills combined.

So really, 180/62.5 = 2.88

180 + 125 = 305/2 = 152.5 /50 = 3.05

You have to remember that the PPS for one of the Full Thrusts would be 180/50 = 3.6 when alternating betwen FT and CT.