Surely the spokes holes you are using are as evenly spaced around the hub and rim as would be the holes on purpose built 18h hub and rim?
The holes will be out of phase by 10° from one flange to the other. You can lace it like an 18H hub, but you will need two different spoke lengths per side.
Means you can't do a conventional crossing pattern though, hence the crow's foot lacing in the picture above.