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Yeah it would be:
X= Stationary Pokemon's starting frame
Y= Number of Roamers
Z= New Flips after substraction
V= Initial Frame
V - X - Y = Z/2
So in your case it would be:
91 - 1 - 0 = 90/2
45 Flips it would be.
Tough if your journal opens and you cancel it make sure it isn't on a...