I did have a discussion about how much a top up valve should fill a tank by. considering it's use cases etc. what it does in real life...
First, the real life example of a top up valve. It is used in cases where, say, you have a tank of rain water to water crops, or your garden, or greenhouse etc. what happens when the tank of rainwater is empty?
You don't want to keep the tank 80% full, you only want to use tap water if the tank is empty, so the real world example of a top up value only fills the tank with tap water if it is below a minimum amount, this case 20%.
as for examples in the game, the biggest use case I could think of was in the Hydrazine production factory. the Hydrazine plant itself outputs pure water, which is also one of the 2 main inputs. there is no fluid void, so you need to pipe this back to the input electrolysers. there is by default no way to prioritise the output of the hydrazine factory over the distillery though. So the use case would be to set up the pure water pipes so that there's a top-up valve on the distillery output, but not on the hydrazine factory output. at 20%, it worked.
Also consider that there may be the case of using a top up valve to fill a tank, but then an overflow valve to vent the gas in it. you don't actually want them both set to 80%.
We discussed different values... 75% (just under 80), 50% and 20%, and to be honest, for game reasons, any one of them could have worked, but I went with 20% anyway.
if we were still running on 0.16 fluid system, where flow is an issue, I probably would have chosen a larger value, but the 0.17 fluid system doesn't seem to care about that, it just takes fluid and passes it from one output to another input.
