I’d say as close to 25 as you can get. once you get it put together you can always rotate your rails to get a good snug fit. Remember everyone is using basically the same exact product. It’s more forgiving than you’d expect but now you see why 25.4 doesn’t work, if you end up with some that is 25.7mm. That is why I try and step in whenever some says 25.4 works fine, maybe they just got a good batch or even a small batch.