Just a quick update, just got some 35mm 29er LB rims, they had an ERD that was 596mm, which is about 7mm out. I bought spokes which accounted for a 3mm difference in ERD, and they laced up perfectly with no thread showing.
The build quality on the 35mm looks excellent, certainly a lot better than I would have expected for £260. They built up incredibly easily, I was concerned with the spoke tension and how much to put through them, but looking on forums LB recommend a max of 180KGf, which is probably way higher than I could even crank the spokes to.