Mutual capacitance pads of footprint component
Draw a precise sketch. Fracture the sketch into tiny squares, with Er indicated in each square. Model that as a grid of resistors, implement a Finite Element Model solution, and you will have the potentials and gradients.

Do the same for a simple parallel-plate model of large area, so the fringing adds little area, and use your answer to calibrate the result of your precise sketch of your 0402.

Note the presence of any SMT component, with its conductive ends, upsets any parasitic capacitance modeling. Thus you must perform the modeling with some assumed installed SMT component.

Additionally, a resistor body, insulating, has a dielectric constant, of unknown value, but different from "air".

And a capacitor body, insulating, has the same unknown effect.