Ok, it's 07:40 AM here in Europe, I just woke up and feel fresh enough to reply:
There are many factors which influence the pitch tuning of a theremin. In case of the Etherwave Standard and Plus models, there are mainly 5 things to consider:
a) Temperature I: these instruments need a warm-up time from 5 to 20 minutes until they really "settle" and no further pitch corrections are needed. During the warm-up phase, the pitch tuning may vary from -4 hours to +4 hours if you see the pitch knob as a clock with its center position at 12 o'clock. What I do to get rid of this, is letting it powered on the whole time. Seen the low power consumption it has no impact on my electricity invoice, but it is always ready to be played.
b) Temperature II: the Etherwaves are sensitive to the ambient temperature. Increasing it by only 3°C or 5.4°F makes that you need to turn the pitch knob about 1 hour clockwise.
c) Capacitive environment: the more electrically conductive material you have around it, the more the pitch knob has to be turned clockwise. Normally, you should create a metal free space with a radius of 2m or 6ft around your instrument to avoid too nasty effects. When you tune your instrument in such a free environment and then introduce a music stand at about 1m or 3ft from the theremin, you'll also need to advance the pitch knob by 1 - 2 hours to compensate that. In all cases avoid to put electronic devices like visual tuners, smart phone etc. on top of the theremin since that will completely kill the pitch field geometry. If you'd like to use such devices during playing, buy a clip which allows you to fix these devices on the mic stand below the theremin where it is much less sensitive. In case of a visual tuner, a short glance downwards will indicate you if you are in key without affecting the pitch field and without the need to bend over the instrument (see next point).
d) The player's position: once well tuned with the player standing at arm length from the pitch antenna (as written in the manual), coming closer or bending towards it by only 10cm or 4inch will also require to turn the pitch knob by about 1h clockwise to compensate the additional body capacitance inside the pitch field. Remember that the pitch field has always a radius of > 1.5m or 4.5ft independent of your individual setting of the zero point.
e) Grounding: Each theremin must be well grounded for stable operation. This is normally done through the connected amplifier's third pin of its wall plug. The Etherwave's power supply has no own grounding to avoid a double ground connection which would lead to hum and additional noise. But an ungrounded theremin (i.e. connected to a battery driven or otherwise ungrounded amplifier) will have a floating ground potential which will make it much more sensitive to environmental capacitance. That means that you'll never find a stable pitch setting.
