I know of several methods of sizing a chain for a bike:
- As many links as the previous chain.
- Small-small. The RD should be barely expanded, the chain nearly rubbing against itself roller-cogs. The logic here is to be able to have the longest possible chain, that still allows shifting into any gear combination.
- Vertical rollers. Shift big front small rear. The rollers on the RD should be in a vertical line, below the cogs. In this position the RD is in a good working point, because it's spring is both tensioned (and thus stiff against vibrations) and not over-elongated (and suffering accelerated wear). This is the medium working point.
- Big-big without the derailleur. Without threading the chain through the RD, measure big-big and add 2 links.
My question is what is the motivation of the last method. To me, not threading the chain through the RD just doesn't make any sense. What physical quantity is being measured by shifting to big-big and why are 2 links being added?
As a side note, I recently tested all of those, and no two returned the same result.