no equation? I think you need to go back to your textbooks!
random offset follows 1/sqrt(AREA). The exact value will be C/sqrt(Weff*Leff) where C is a constant for your process including Tox and Weff and Leff are the effective W/L of your device.
So thinner Tox gives better matching with a linear dependence, and larger area gives 1/sqrt() better matching. I usually say 1um square gives about 20mV random offset in a 1/2 um process.
You will have offset due to the diff pair and any sets of matched current mirrors. Cascodes usually aren't too bad. To calculate, you can probably estimate only the first stage. Diff pair will have a delta-Vt based on area, and mirrors will have a delta-I. Convert this delta-I to input referred offset and add to the delta-Vt of the diff pair, and that's probably a reasonable estimation.