Simple rule of thumb for 350 is diameter + 3 for top and diameter - 3 for bottom. And yes, there is an app for that
I've used this for many years and I have also checked it out with a remote thermometer. It will be very close if the ambient is around 60 to 80 degrees. If the ambient is more or less the difference isn't that great, but enough to affect the time some.
a coal on top and one one the bottom will give you 20-25 degrees on a CI dutch.
If the dutch is exposed to the wind everything goes haywire.
2009 Dodge 3500 Laramie, DRW, 4X4, auto, 6.7L, B & W Companion.
Jayco Designer 34RLQS, Mor/Ryde