Not sure about the maths there. For me it’s an all or nothing swing. If you play Murphy and he comes on for a 30 you lose the difference between his score and that of the player you’ve shifted to the bench (in my case Jones). So if Jones scores 60, you lose 30.
On top of that, you lose the difference between 30 and the score of the player you were going to captain. So using your example, if Grundy scores 120, you lose 90. So that’s a total loss of 120 v upside of 84 in getting Oliver’s 204 v Grundy.
If you’re benching Ridley, then the downside is even worse.
Basically there’s a lot more to lose than gain, so if you’re at all risk averse, you kinda just gotta suck it up.