I bought myself a 74mm o.l.d. dynamo hub that's 24 holes, and my current rim is 20 holes, I like to lace it radially, how do I go about calculating the spoke length and where do I skip a hole on the hub?

Normally I'd just go out and buy a 24 hole rim but aluminium 24 hole rim in 305mm (16") rim size is extremely hard to find, so I figured lacing up a 24 hole hub to a 20 hole rim and skipping 4 holes is much easier.

For those wondering, this is for a folding bike btw.