Disney runs on ratios in three phases, the phase level depending on demand (basically, how long the Lightning Lane is/is expected to become in the imminent future).
The numbers may have been tweaked since the Genie+ changes, but that’s not my understanding.
Phase 1
For every 4 standby guests you admit up to 16 Lightning Lane.
Phase 2
Depending how critical the situation is becoming, for every 4 standby guests, admit up to 56 LLs, though it can be lower (mid-20s I believe).
Phase 3
For every 4 standby guests, admit up to 40 LLs.
As I understand, Phase 2 is intended to try and quickly flush the Lightning Lane and restore normal Phase 1 operation. Phase 3 is for more sustained crowds that are unlikely to reduce down, which is why Phase 2 can actually be more demanding if it needs to be. Cause a bigger disruption but for a shorter period of time to get back to normal quickly, and avoid a long-term Phase 3.
However, it’s also important to note that Genie is intended to prevent sudden surges by spreading out the return times and limiting the number of people joining LL at any one time. This means under normal operation you shouldn’t really see queues forming at LL. As a result, standby will flow freely until someone comes down the Lightning Lane. That group will be admitted and normal flow will return. So while Phase 1 can allow up to 12 LL to 4 standby, in reality you might see the inverse, where standby is feeding the majority of guests. You don’t block the standby line and wait for another 6 LL guests to come along.
We recently used LL on Slinky Dog Dash, a ride that’s famous for selling out in seconds. We were the only people using the lane when our return time came up, so aside from us 3 slotting in, standby was using the full capacity of the ride at that point. It’s only usually when there’s downtime and untimed recovery passes are issued that you start to see lines building up. That’s when the phases step up to try and clear the backlog.
The merge phases are also monitored and controlled on the majority of Disney rides, with the screens at merge able to tell cast what phase they should be running at and tracking numbers. There’s always going to be a human element with staff discretion, but as a system it’s far more regimented. As others have said, at Chessie there’s a huge demand coupled with low throughput and the complaints at merge. It’s not surprising that staff may panic and try to run down a line that is constantly growing. Chessington seems to run in a constant Phase 2/3 state for many of its rides.
You can only really run a fastrack system well if you have strict controls at every point of the journey (from point of sale/issue, to sitting down on the ride). I’m not saying Disney is perfect, far from it. As a standby guest, phase 2 and 3 can be frustrating. But they have far more control over their system than Chessington.
