I've just put together a table for reading off mode 0 screen bytes. I'm sure most people have calculated one out before but it may be useful to someone. Things seem to be correct although I haven't double-checked everything by hand (the whole point of the table was to avoid doing this ::) ).
Read the left pixel pen number in the leftmost column and the right pixel pen number in the topmost row, then cross reference your corresponding byte.
I'm not very good at these 2D Chart Layouts. :(