Depends on the task but it’s the volume of gas in the pipework your working on
As for how to work out the volume it’s the size of pipe find the volume 1m holds in your gas book then times by the amount of pipework in your section or divide by the volume this will give you a meters answer etc...
Yes because once you have the base fitted at the time of the toilet install the flapper is just clip on
I’m guessing yours attaches with glue / sticky pads ?
Fluidmaster already sell one that you can change in under a minute also something isn’t totally universally drop in
https://www.fluidmasteruk.com/products/pro-dual-flush-flapper-valve-1-1-2-flush-outlet/
So what’s your product?