I know that you're asking about an X-Kilt, but I do want to stick my oar in with respect to what you said about measuring for a trad kilt. Every kiltmaker does this measurement his/her own way and then uses the measurement differently, so there's no "consensus" for a trad kilt. The ONLY sensible advice is to follow the instructions that your kiltmaker asks you to use. If you don't, you will likely wind up with a kilt that is the wrong length.