Some of the spoke calculators have rim and hub databases, but I'm not sure if they include 27" rims. Plus, it is always good to do your own measurements.

For the hub, a caliper is handy. Harbor Freight has reasonably good digital metal calipers (inch and mm) for fairly cheap.

https://www.harborfreight.com/6-in-d...per-61585.html
https://www.harborfreight.com/6-inch...per-47257.html

Hmmm, I thought it was $9.99, but I think that is with a coupon that comes up from time to time. However, you can at least get a 20% off coupon.

https://www.harborfreight.com/digitalsavings.html

Anyway, with the calipers, measure the flanges and flange spacing as good as you can. Diameter of the flanges is most important. Otherwise, you can normally just be close for the offsets.

For the ERD, I usually measure my nipples, then assemble nipple (washer?), and two spokes on opposite sides of the rim. Tie the spokes together with a rubber band, or hook the J-bends together, then measure from the base of the nipple to base of the nipple, and add twice the length of the nipples.