Let start from smaller time divisions and work our way to larger ones, shall we ? (we could just as well do the opposite but we need to pick one way.)
First of all, the time signature shouldn’t affect your LPB, so you can set it to anything you want, once for the whole song. In 6/8, a beat is a 8th note. If you want to play 16th notes without the help of delay commands, and since a 16th note is 1/2 of a beat, then set your LPB to at least 2.
Ok, with that we have consistent beat division. Now we need 6 beats per bar.
Renoise doesn’t consider bars as a meaningful time division, but you obviously would like a practical amount of bars per pattern: probably 1, 2, 4, 8 or 16 bars per patterns, unless you’re doing more complex structures. All you have to do for that is set your patterns’ length to the number of bars you want per pattern, multiplied by 6 (the number of beats per bar), multiplied again by your LPB.
Here’s an example: say you want 2 bars per pattern, 6 beats per bar, and 2 lines per beat. Then set your LPB to 2, and you pattern length to 2x6x2=24 lines.
You could also consider that a beat is a 4th note (not a 8th note), which would be more orthodox, and your signature is equivalent to 3/4. In that case, the above example becomes: 2 bars per pattern, 3 beats per bar, and 4 lines per beat (there are 4 16th notes per 4th note). And 2x3x4 still equals 24. The only difference is one line out of 4 will be highlighted instead of one out of two - it’s up to your preference.
Now with all these ratios, maybe at some point you’ll need to set your BPM to 40 or 160 in order to get 80. I can’t say because right now my brain is smoking, but it wouldn’t surprise me because we’re playing with 8th-notes as beats in some cases, and I believe a metronome always consider a beat to be a 1/4th note. I’m not sure and I need to lie down right now, but you will easily find this out: if your songs plays twice too fast or twice too slow, multiply or divide the tempo by 2.
I hope it helps (and I’d like to know if it works).