I've used this DT calculator for many years with good results. While I don't usually build radially spoked wheels this calculator does have a "straight pull" hub option (as well as nipple options). Do know that radial spoking has less "fudge factor" in the inputs to get the best length. Exactly where on the hub I would call the flange diameter I am not sure... Spoke calculator - DT Swiss

Having known spokes in use makes the reversing math to get flange diameter easier. I pretty much will always trust a spoke in use before a calculation, WRT lengths for a specific wheel. Andy