I just clamped a long spirit level to the spindle and measured how much it diverges from parallel with the side of the chassis. I cut shims out of a beer can. The can was 0.1 mm thick and each shim was worth 0.1° or so.
Simple trig: If I get 8 mm divergence over 550 mm, the toe for that wheel is arcsin(8/550), that's approximately
8 / 550 * 180 / 3.14 = 0.83°
8 / 500 is really the sine of the angle, but in the vicinity of 0 sin(x) is close to x, then 180/pi just converts radians to degrees.
P.S. Toe in millimetres means the slant between the front and rear of the tyre of some unspecified size. Sierra tyres are about 57 cm in diameter. You can use the same trig to convert that into degrees.