Originally Posted by
joejack951
If you read at that link, you'll see that the ideal chainline measurement for your application is going to be between 43.5 and 45 (double and triple chainline specs respectively). If you are using a track/singlespeed crankset intended for a 120mm spaced frame, take the manufacturer's recommended spindle length and add 3-6mm (verify that the spindle you have chosen is symmetrical first). This should center you on the cassette.
I just wanted to add that everything written here is correct. However, a lot of the cheaper single speed cranks are NOT designed for true 120mm track frames but are instead intended (as far as I can tell from reading the specs) for 130mm conversion/IGH hub applications. So, don't assume that a crank is intended for 120mm spacing without carefully scrutinizing the specs.