i know that the soundmen like to get signal between -12db to -18 db in digital mixers and signal about 0db in analog mixers.
How can I know how much db my kemper send out to the mixer? maybe too loud or too low?
How can I measure it?
Yep, peaking around -12 dBFS and averaging or -20 dBFS is a good starting point when mixing. It’s laudable you try and get things to a sensible output level prior to arriving at a gig. There are many variables though between live sound rigs that will mean you’ll always have to talk to the engineer and they will want to set levels based on real signal. For example, the analogue operating level of different desks can vary by a significant amount. Also, you’ll have almost exclusively mic inputs on-stage, meaning the engineer will PAD down those that you feed with your Kemper, which outputs line level. PAD functions on different desks and their associated stageboxes vary. Many are -20dB but you’ll find -15dB and -30dB. This means the mix engineer’s workflow will be to pad, then increase input gain until they meter where they want them to. I think the most important thing to tell your mix engineer is that you are outputting line level, not mic level, so that they can be prepared.