While the formulas for combined events are easily obtainable in the literature, the general IAAF points for athletics that are used to compare performances across events are only published as lookup tables. Here is the math behind those tables.
It's simpler to grasp this starting with the simpler cases.
In track events the result is measured against a reference time. Your improvement with respect to the reference is then squared and multiplied by a certain factor that converts squared seconds to points.
Let's consider 100m outdoor men as an example. The reference time for this event is 17 seconds in the 2017 edition.
Suppose John ran the distance in 11.78 seconds. That is 5.22 seconds better than the reference time. John's result is equivalent to 24.63 * 5.22^2 = 671 points (we round the decimals down to the nearest integer). The 24.63 is a coefficient specific to this event.
The formula for track events can be expressed like this:
points = floor(conversionFactor * (reference - result)^2).
The other events have one more event-dependant coefficient.
First, the result is shifted by a number (similar to comparing with reference in track events).
The shifted result is then squared and multiplied by a factor. And this is then shifted by another number.
The formula can be expressed like this:
points = floor(conversionFactor * (result + resultShift)^2 + pointShift).
This formula can (and is) also be used for track events by setting
resultShift = -reference and pointShift = 0.