People assume the ratio of FP distributed is static - it's not.
For a normal day there's an amount set aside per time slot that's typical. It matches the ratio that operations will use as a baseline when merging the two lines.
In the daily ops side of things, the merge ratio will increase to allow more FP guests if the FP line begins to back up. There's a series of phases - but the worst of it was used at Toy Story where one standby guest is allowed per every 100 FP guests. Something like 1:10 is a busy number for most rides. 1:4 or less is common as a baseline.
Similarly, on the backend they planned days based on expected ride capacity (number of vehicles online) and staffing and guest projections. Toy Story would sit around 80% of capacity being dedicated to FP. But when it got really busy itd climb up to 90% and death-con "planned" 95%.
When the changes in those numbers are made for a day, more FP will be available.