Note that John Citizen is in Australia and a lot of the time fuel for an alternate is not legally required at all. You can be planned to a single runway and provided the forecast is ok then you can plan to get there with fixed plus variable reserve and burn through the variable reserve on the way.
As for what you should actually carry? Our company policy is to plan to land with 2000 kg (about an hour of fuel) assuming no other requirements. On a nice clear day/night I‘m totally happy with that and am comfortable burning into it a little bit. On a typical sector, if we were to plan minimum fuel we’d be landing with about 1200 kg including variable reserve. To my mind, that’s not enough, it only takes a single go-around and you’re looking at eating into fixed reserves, but 2000 kg, no problem.
There are a few times I would plan to have more than 2000kg. First, when it is required by the company fuel policy, i.e., weather below the alternate minima so an alternate is required and/or thunderstorms or traffic holding requires extra. Second, when it is not required but I‘m not happy. An example is when fog is forecast for after our arrival time. I have seen fog come in early often enough for that to trigger alarm bells in my head. When I want extra fuel but it is not legally required, I will discuss with load control to see how much can be taken without affecting payload. I may decide to take as much as that allows as that gives me divert options for as long as possible, or I may decide it’s worthy of offloading payload.
The reality is, particularly on the East coast of AUS, even if you don’t have enough fuel to get to an alternate, the amount of time you are committed to your destination is normally minimal.
In summary, I’m happy to go along with company policy, but if ever I feel like I need more, then I will take more. I’ve never been asked to justify my fuel load, but if I was and I couldn’t justify it, then it was a poor decision. In other words, I take extra when I have a reason to.