Colin's thread states that standard wheel studs are 25mm
The standard wheel studs are 25mm long. That means they will protrude 5mm through a 20mm spacer, but there is just enough room inhg the spacesaver design to accommodate that 5mm protrusion and allow the it to fit snugly on the spacer.
Just to clarify - whilst the spacesaver will fit on 20mm spacers, I found the OEM 370 front wheel design would not accommodate the 5mm stud protrusion and that a minimum of 23mm was necessary to allow the wheel to seat squarely on the spacer(s). Hence the recommended choice of 25mm spacers, avoiding the use of slip spacers that were found to be responsible for disc distortion due to poor quality.