This means realFluidRate = tooltipFluidRate * 20 / (tickRate * tickRate), which is 4/5 of the fluid rate shown in tooltip in default settings.

I don't follow the logic here. The tooltip will show both the effective rate (transferRatePerTick) and also the rate per graph tick (which is that formula there). This is done because conduit graphs tick only 4 times per second.

If I'm missing something obvious though do point it out :)

Because ticks 4 times per second but the graph tick rate is 5 (20 / 4).
For example of the lowest tier of fluid conduit, its extraction rate is 50mB/t which equals to 1000mB/s. But using the mentioned formular, every graph tick only transfer 200mB, with 4 graph ticks per second, gives a total 800mB/s.

The correct formular should be int rate = (int)(conduit.transferRatePerTick() * conduit.graphTickRate());

Aha I see! This code is a hangover from when transferRatePerTick was perSecond. Thanks for catching this - I'll get it fixed up :)