I assume you want to work out the minimum value for a pullup resistor that will give a voltage below VIL for an IO pin in open-drain mode.
That is not a simple task, chiefly because you don't know the ON resistance (RDSON) of the low-side MOSFET in the chip.
It's possible to come up with a rough estimate (see here) though.
With VIL of 1.5V and maximum current of 40mA that's an estimated theoretical resistance of 1.5/0.04 = 37.5Ω.
Note: this is just an estimate based on the absolute maximum and DC Characteristics values from the datasheet.
So if we take that value as actually true you can work out the upper half of a voltage divider that would result in 1.5V coming out. There's two ways of doing that - using the resistance or using the current.
Taking the resistance R1=R2(Vin/Vout-1), 37.5 x (5/1.5-1) = 87.5Ω.
If you want to do it with the current (40mA "absolute maximum"), then R=V/I = (5-1.5) / 0.04 = 87.5Ω.
However, that comes with a big caveat. The IO pin, while it can, for short periods, sink 40mA, it's not designed to do that for long periods. For that 25mA is the recommended maximum. So a resistor should be chosen that keeps the current below 25mA.
Assuming 37.5Ω for the MOSFET resistance, and 25mA through it, that gives a voltage drop across it of 0.94V (well below VIL). The upper resistor in that divider would then be (5-0.94)/0.025=162.5Ω
So to sum up:
- The absolute minimum resistor would be 87.5Ω
- The recommended minimum resistor would be 162.5Ω
- The resistor you would choose to use would bear no real relationship to those values since you would normally choose something more sensible, like 3.3kΩ unless you wanted to run the I2C bus at insanely high speeds or with massive load capacitances.
You really don't need to know the theoretical minimum. You know the maximum already, and all you really need to know is "is the value I'd like to use safe for the IO pin" - i.e., does it draw less than 20mA?
For that you can ignore all the other values. All you care about is your resistance, the Vcc voltage, and the resultant current.
For instance, for a 900Ω resistor with 5V across it you get 5.6mA. You know for a fact that you can never have more than 5.6mA regardless of the resistance of the MOSFET - it can only go down from there.
5.6mA is below the recommended maximum of 20mA, so you're perfectly safe with that resistor.