i figured this out yesterday, and i bet a bunch of people do something similar, but this is for people who don't know:
Before i use to look at powertab solos and see, "hey how did they come up with those odd rhythmic groupings? quintuplet, septuplet, 7:6, 9:6? what the hell?". I didn't even know that 7:6 or 9:6 meant.
I've tried powertabbing fast solos, and the measure would always be underfull or overfull, because i wasn't guessing the right speed at which the notes were played. The speed of the notes determined how long the notes were, and if you got that wrong, then you'll have the problem with your measures being underfull or overfull. On top of that, I couldn't find where the solo started with relation to the rest of the riff, so my solo could've been displaced a few notes to the right or the left, which would also make it hard for finding the right amount of notes in a passage.
So i figured this method, and it has made things somewhat easier:
1. First, determine the bpm of the song. this is very important.
2. Go into Cool Edit ( or a similar multitrack and wave-editing program) and put your mp3 into multitrack view. Select one measure of the mp3, try to find where the riff starts and ends before it repeats. (Split this into another track if you need to , that'll help). Take a note of how long this passage is in seconds.
3. Go into Powertab, and set the metronome to the correct speed. Go into guitar setup and change the instrument to Woodblock (or whatever instrument you can hear easily over all the instruments in the song). In the measure,put one eighth note down (I put it on the B string, 0th fret). Export this eighth note to midi.
4. Import that midi you made in step 3 into CoolEdit onto an adjacent track.Line it up with the beginning of the riff you want to transcribe. now loop duplicate the midi 7 times. These 8 midi blocks next to eachother should make one big block, and should be the same length as the riff you want to transcribe. This is crucial, if they're not the same length (or pretty close), go back to step 3 and adjust the bpm.
5.Now with these midi blocks, you have the riff subdivided into 8 sections, so you can figure out what the guitarist is playing in each 8th note of the passage! All you have to do is select and drag how many eighth notes you want to hear (the selection will snap to the midi blocks) and hit play so that the metronome midi block and the passage you're trying to figure out play simultaneously. With this you can figure how many notes are in each eighth note, and where the note starts in relation to the other notes. This will help you figure out those nasty tuplets
if you select 2 eighth notes and play, let's say you hear 5 notes in that span of 2 eighth notes. 2 eighth notes = 4 sixteenth notes. That's 5 notes in the space of 4 sixteenth notes, i.e. a quintuplet. put down 5 16th notes in your powertab, select those 5 notes, hit 'i' on your keyboard. you'll probably see 6 in the space of 4, so just change that to 5 in the space of 4. and there you go, you just figured out a quintuplet
same thing if you heard 7 notes in 2 eighth notes, that's 7 in the space of 4 sixteenth notes, that's a septuplet.
if you hear 3 in the span of 1 eighth note, that's a 16th note triplet.
if you hear 6 in the span of 2 eighth notes, that's a sixtuplet.
now what if you notice in the 2 eighth notes, that the guitarist plays a lick that's 4 notes and the last note of that lick stops in the middle of the 2nd eighth note. if he played those 4 notes in 1.5 eighth notes, that's 3 16th notes. so then if you pressed 'i' you would get a funky "4:3" over the notes, 'cuase it's 4 notes in the space of 3 16th notes.
that's how you get funky groupings of 5:6, or 8:6, etc.
And since you're specifically looking at each 8th note, you won't go over or under the measure and get frustrated at why you suck at tabbing so much
6. put the cursor line (the vertical one) at the end of the big midi block. group the midi blocks together and move them to the right with the right click button. and then repeat step 5, and eventually you'll have the whole solo figured out.
if you want me to upload screenshots to help you, i'll do those upon request.