It is usually 2.6mm. W, width from center to flange may differ between left and right sides of the hub. In this illustration, W L and W R are the dimensions you enter into the spreadsheet for the left and right sides of the hub, respectively. A few millimeters error in width will only lead to a fraction of a millimeter error in spoke length. Of course, I use two hands for better accuracy.
Most people prefer it near the end of the spoke nipple.
Count holes to be sure. Screw some nipples onto the spokes. Do this at several spots around the rim and average the measurements. Of all the dimensions you actually might measure, ERD is the most critical dimension affecting spoke length, so it makes sense to measure it a few times at different places around the rim. Off Center or Asymmetric rims require a slight adjustment for spoke lengths.