The issue is that these boundaries should be changeable, since different chars will have different pixel width.
There are two ways of doing this - all capitals have same width and all non-capital letters have another, smaller width. This indeed reduces some visual gaps, but not exactly what I want.
So my idea is to have a spare, "mask" byte for each character, which will define it's width and entry and exit points.
Say, there's "t", it is 4 pixels wide, and "mask" for it will look like this 00011110. zeros will be trimmed, and bits #5,4,3,2,1 from the letter shape will be added to output image.
Another, more complex idea, is to do it "ZX Spectrum way", via complex STR/MID/LEN/ASC and other statements, currently not implemented, but still doable, in PBP....