The reference document for this is PCI-E Card electromechanical specification. I believe the relevant figure is 6.2:
As you can see, the distance you're asking about is not specified at all, because the lug (referred to as "detail F" in the figure) has no connectivity and is present to prevent insertion of PCI-E cards into PCI slots. It doesn't have to be positioned at a precise distance from the bracket for the card to work.
What is specified is the distance between the card bracket and the notch inside the connector, which is 59,05mm.