Use the figure as measured, then round down on the spoke lengths. LB seem to measure just to the inside of the rim, without allowing for the part of the spoke which should be inside the head of the nipple, so assuming you want the spoke to be roughly flush with the top of the nipple you would expect to add a few mm's to their measurement to get the true ERD.
They do, although the 'not including nipple head dimension' could be a bit clearer as to what they're talking about. Once you know it makes sense, but it's not exactly obvious to the uninitiated.
Use the figure as measured, then round down on the spoke lengths. LB seem to measure just to the inside of the rim, without allowing for the part of the spoke which should be inside the head of the nipple, so assuming you want the spoke to be roughly flush with the top of the nipple you would expect to add a few mm's to their measurement to get the true ERD.