This is how I work (hopefully others will join us and share their methods):
1 - Download the -18dBFS tone from the attachment below.
2 - Calibrate your host meters with this signal.
3 - Insert a Nebula instance and check if "4 RMS 17" is set as metering scale.
4 - Check Nebula's meters:
5 - Done, keep your tracks at this value (or below).
After some tests I've come to the conclusion that a safe value for tracks is -12dB or -11dB (that's why I suggested to calibrate your host meters in the first place), look here, this is an example with Reaper and calibrated meters:
To keep my tracks within this range I use 2 plug-ins, before and after each nebula instance (in my case, using Reaper, I use the JS effect "volume_pan"). I use 1 plug-in to control the signal that's going into Nebula and another one to compensate if needed (this is what we call "gain staging"):
(click on this picture to enlarge and see all meters and descriptions)
-18dBFS as average level of audio signal is an European standard and means 0 VU. Nebula works like a analog hardware so use standards of the analog world > -18dBFS = 0 VU. If you work with 32/64 bits float system is almost impossible to distort but at the very end you must do a DA to listen into yours speakers in 24 bits fixed, so an average level of -18dBFS is a safe value.
Enrique Silveti. Acustica Audio customer and technical support.