24mm = 1" 1/2" is 12/13mm so it is not always an exact science. it really is not 24 but something slightly larger 25.4mm = 1" to be exact. so that ET is calculated based on the 1/2 measurement of the total width of the wheel compared to or subtracted from the actual backspacing measured. In the example 6" was the middle point on the 12" wheel if you had a actual measured backspacing of 7.5" - 6" that is where you get the 1 1/2" and convert to mm to know the ET.
I just built a 10" front wheel -- 4" outer rim, 6" inner rim and low pad height and in the end the measurement was 10 ET 10" as measured. I built a 13" rear wheel. 7" outer rim 6" inner rim and when it was all said and done it was a -24 ET 13" wheel. Jim
jdorociak@gmail.com