I usually fly with zero percent nitro at sea level. What would be a good nitro percentage for flying on 400 meter altitude to achieve the same power?
"400 meters" is about 1300 feet, I would suggest 10% to start. If it runs short on the same volume of fuel, go down to 5%. The one hedge I will have is that using any nitro at all will greatly improve the power so you may have a wildly disproportionate effect of even tiny amounts. If you baseline 10%, changes above and below that are generally pretty linear.
You should be able to tell immediately if you get it right, because you will not need to move the needle very much to get the same RPM and the same "setting" (i.e. it will be running at the right speed at the right point relative to the 4-2 break point). You *should not* have to change the head shims, venturi, etc ,since all you are trying to do is replicate the same combustion conditions. Obviously, if it starts detonating or acting "fierce" then you will have to add a head shim, but no more than .005. But I have run plenty of FAI-fuel engines on 10%+ with no real issues and stunt engines are generally very conservative to begin with.
If you need some intermediate value between 0 and 10, you can mix it in the syringe. Say, if you are want to run a little less than 10, pull 1 ounce of FAI fuel into the syringe, then pull the rest from the 10% can. Since you really don't know what you need, it doesn't really matter if you know the exact percentage. People have in fact done this *on every flight*, where then more-or-less never change the needle and just adjust it for temperature/pressure/altitude by changing the mix based on experience, as an alternative to needle adjustment. I haven't tried to do that, I always reserve the possibility of fine needle adjustments, but over a very narrow range.
For reference, this is what I use for various conditions:
Sea level, 50ish degrees 10% with a pitch reduction (3.6 VICE 3.75)
Sea level, 70-110 degrees, dry, 10%, 3.75" (baseline)
980 feet (Muncie) , 70-100 degrees, humid, 15%, 3.75
2200 feet (Tucson), 70-80 degrees, dry, 20%, 4.1
You might need different corrections but the general trends should be clear. 1300 feet is definitely enough to warrant a change.
Brett