Calculate the parasitic capacitance in Altium

I assume you are looking for a schematic symbol, and perhaps footprint. If so, you should just make the required symbols yourself - then you will know that it matches the component you are using (and, for the schematic symbol, that it has the pins arranged (and named) in a way that will suit your use.)

I don't trust other people's libraries (even Altium's - I once found an IC symbol that bore no relation to the maker's datasheet!).

There is much misinformation out there on this. The load capacitance specified for the crystal is what should be across the crystal from the crystal's point of view. As a first reaction, that is the series combination of the two caps C2 and C4.

However, there is a lot more going on there and various (usually invalid) assumption are embodied in this common knee-jerk formula. Instead of rules of thumb, look at what is really going on. Two things that can significantly change the apparent capacitive load across the crystal are stray capacitance and the impedance of the driver driving the input signal to the crystal.

Consider how this circuit looks if the impedance of the crystal driver output is 0 (perfect voltage source). The cap on that output is irrelevant, and the load seen by the crystal is the full output cap. To get some idea what impedance range this is relevant for, consider that the impedance magnitude of 22 pF at 16 MHz is 450 Ω. Is the impedance of the output that is driving the crystal 450 Ω or less? Quite possibly yes. However, this is rarely specified, so you have to guess.

You also have to guess what the stray capacitance might be. I'd say you're 5 pF figure is reasonable, given a reasonable layout. So if the crystal driver has infinite impedance, the capacitive load on the crystal is 16 pF. If it has 0 impedance, the load on the crystal is 27 pF. The reality is of course somewhere in between.

In reality, for normal typical microcontroller crystals somewhere around 10 MHz (factor of 2 is close enough), 22 pF caps on both sides is a good bet.

The point to all this is that there are a lot of unknowns you can't account for. This is one place you can't analyze down to small detail. If two 22 pF caps seems to get you in the ballpark, just go with that. That's what I do, and I haven't had any problems. Fortunately crystals have a very sharp frequency curve and a factor of 2 either way from the specified load capacitance is usually fine. If you need better than a few 10s of PPM, then you probably need to do some tests with carefully calibrated instruments.