ET vs Backspacing, What the... Options

[Krieger]

I am going to order some new wheels for my track car. 15 x 9. I understand backspacing. I think all 911 and 914/6 backspacing stock is 125mm. That is what I'd like. The manufacturer is listing three different ETs: 3mm, 10.6, 23.3. Which ET gives me the stock backspacing?

[SirAndy]

I am going to order some new wheels for my track car. 15 x 9. I understand backspacing. I think all 911 and 914/6 backspacing stock is 125mm. That is what I'd like. The manufacturer is listing three different ETs: 3mm, 10.6, 23.3. Which ET gives me the stock backspacing?

ET (Einpresstiefe) is the distance from the center-line of the wheel to the mounting surface.

It's normally measured in millimeters and is negative if the mounting surface is closer to the inside of the wheel and positive if closer to the outside of the wheel.
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PS: To get backspacing use simple math: (wheel width / 2) + ET
PPS: Of course, make sure you convert all variables to the same system first, either imperial or metric.
PPPS: Don't forget that ET can be negative!