In the manual it says the meausurement on the instrument envelope x-axis represent ticks. I have a song with 4 LPB and 12 Ticks Per Line. That should mean a beat in the envelope editor is 48 ticks long right? But it is not and after manually figuring it out a beat corresponds to 23 (?) ticks for this song. How can you determine the length of a beat in the instrument envelope without trial and error?
The info is outdated. Try making as tight triangle wave amp envelope as you can with some static sound, put LPB to 4 and TPL to 3. What do you hear? Do you hear?
That is because your envelopes has static resolution of 24 per beat (count zero as a number, 0 to 23 equals 24 values), so if you do 34 ticks per beat you hear every 24/(34)=24/12=2nd envelope position. Which means every second position of envelope is sampled.
If you double the resolution with LPB or TPL, any other of which doubled leads to doubled resolution, you hear every envelope position but no more, which means your beautiful triangle envelope has become a bulky square one, because there are no values sampled in between lines.
On your example you have 12*4=48 ticks per beat, so your automation samples every 24/48=1/2th line of you instrument envelope. In other words there is two sampled values in one automation unit.
It might seem like a bit difficult at first, but allows all kind of whacky effects if abused a bit, since TPL*LPB is the rhythm that all modulation and automation in Renoise follow.
Well, actually it is displaying beats, the bars are 24 units from each other until you hit the 496 lines mark. Additional unit types could be nice, but there is new XRNI format to come, so dunno how eager they are to improve current one. But you are welcome to post the suggestion, just remember that ideas & suggestions forum is pretty crowded nowadays so make sure you write a good argument to make a point clear and rise some discussion. Two-liners rarely gain very much popularity there.