Looking for a really easy and simple spoke caluculator,

wanting to build up some wheels Hope pro 2 front 15mm, and rear 12×142, rims will be Mavic en423s

I use DT Swiss Spocalc – never let me down. You’ll need to know the hub dimensions as well as the rim ERD – all info you can find online.

http://www.prowheelbuilder.com/spokelengthcalculator

Just check the nipple length. caught me out a couple of times, CRC were selling one with a short nipple length and my spokes came up short.

Nipple length doesn’t affect accurate spoke length, the correct length will come through halfway through the top of the slot in the nipple, regardless of the nipple length that remains the same.
Anyway, in terms of a spoke calculator, I use the Wheel Pro one & seems spot on.
ERD’s aren’t always as accurate as you’d expect either & can vary around the rim, so measure it yourself in at least three points & take an average.

This one also works
https://leonard.io/edd/