One thing I'm a little stumped by in the app is figuring out the heights for the fixed pressure readings. They only record 3 digits of the heights. So if it says 850m, it might actually be 1850m, but I can't see any way that you know it is 1850 and not 2850 (you'll probably know it can't be 850 because you'll have seen a 900 before).

Can anyone help me out with this?

This is what I'm reading on it (3 different sources):

1:
P1P1hhh...the first mandatory pressure level data follows, with
00 being 1000 mb, 85 being 850 mb, 70 being 700 mb, etc.
hhh is the height of this level in meters above sea level.

2:
the standard isobaric level height is listed, in geopotential meters. At and above 500mb, heights are in decameters, with any thousands or tens of thousands omitted. For 1000mb and 925mb heights than are negative, 500 is added. When the pressure is less than the isobaric level, slashes are encoded. (SLP = 950mb: 00/// ///// /////.)

3:
h1h1h1 â€“ Heights of the standard pressure level in geopotential meters or decameters above the surface. Encoded in decameters at and above 500mbs omitting, if necessary, the thousands or tens of thousands digits. Add 500 to hhh for negative 1000mb or 925mb heights. Report 1000mb group as 00/// ///// ///// when pressure is less than 950mbs.