The answer really depends on several factors; The important number is really "Density Altitude "
Howard would like to have a word with you....
which is effected by temperature, altitude relative to sea level, barometric pressure, and to a small extent by humidity. A small change won't have much effect but a big swing will definitely change the needle setting required to achieve the same type of run.
I snipped the rest of it since it was a good overview. My point was that the barometric pressure just can't change all that much compared to the other factors. If you want it in terms of density altitude, it might make 500 or so foot difference - not nothing, but also, not to the point that a needle tweak won't correct for it. If it was me, and I wanted to really hi-fi it for 500 feet change, I might go from 10% to 12.5% nitro just to get the needle back.
That also shows why "density altitude" - a pilot's rule of thumb - is not particularly representative of the performance change. Going up in actual altitude/pressure altitude by 1000 feet has a huge effect, whereas going up in "density altitude" by 1000 feet due to temperature - 15 deg F - is something you can almost ignore. We routinely deal with 2500+ change in "density altitude feet" with no changes aside from half a needle width tweak. When I go to Tucson, actual altitude 2200 feet, I run YS 2020* instead of 10% sport fuel, and even then, have to add some pitch as well.
Now we know Frank *does* have a pretty workable and tolerant engine, a *stock* 46LA, that even from the worst-case pressure change it should be a few clicks of the needle. This strongly suggests something else was different, because engines don't just run drastically differently or fail to run at all just from the barometric pressure changing over the normal range for the continental US.
Brett
*the sound it makes running YS2020 is utterly indescribable - not loud (actually probably quieter on a meter than 10 or 15), but completely unique.