I’m thinking of getting my Hope Pro3 AM4 hubs built onto some LB 33 or 38mm carbon rims. The ERD of my current Flow’s is 536mm, the LB’s are 515mm (33mm) and 510mm (38mm).
My LBS wheelbuilder has worked out hat I need 248mm spokes for 33mm wide (old ones are 266mm).
I make it 256mm by subtracting the differnece between the ERD’s but I know faff-all about wheel building 😳
Does 248mm sound right and, if so, any ideas where I can get some?

Did you use a calculator?

https://leonard.io/edd/

You don’t just difference the ERD, the spokes don’t go radially from the hub.

Also, I’d be waiting till I had rims in hand to measure the ERD accurately till I ordered…

(and then use one of the many spoke calculators out there, remembering to account for nipple length)

My hub isn’t on there?

This is the calculator I use: http://www.wheelpro.co.uk/spokecalc/

There’s instructions on there for measuring as well, which is always better than relying on someone else’s numbers. Output from that gives you lengths for different crossings and also “perfect” and “OK” lengths, depending on what’s available.

I use the Spocalc spreadsheet it’s getting on a bit now, but has the distinct advantage that you can add data for any hub or rim, I normally scour the web or measure parts and then I can work out spoke lengths…

Plus you can save it/use it offline, it’s very handy IMO…