I am attempting to build up an old left-handed Araya rim (the unmarked 27x1-1/4 ubiquitous alloy one, with the slight rise in the midsection, for which there is no model designation ).

The extreme outer diameter of this rim is 640mm - and the depth of the flange (to the "floor" where the spoke nipples seat) is 10mm.

Since ERD information for the rim does not exist- I calculated it by taking the O.D. and subtracting the depth of both flanges, or:

ERD= 640mm - 2(10mm) = 620mm.

The resulting spoke length with a Normandy High Flange front hub comes out to 296mm (which seems reasonable for a 3-cross pattern) - but before I order new spokes, can anyone confirm if I calculated the ERD correctly?