Here's how to build a MIDI tempo map in n-Track, to an existing
song that was recorded without a steady click track.

It's tedious and a little time consuming, but in an hour or so
(once you get the hang of it) you can do a tune where there's
a LOT of variation.  Then you can MIDI drum to your heart's content.

Before you start: Set the grid to "Measure".  Set the grid view to
"always on".  Turn the grid off.

These next steps are just to set the BPM to start with.  If you
have a tool that calculates the average BPM (i.e., as mentioned
above), just use that instead -- it's probably easier and no
better or worse.

1)  Find the first beat in the first full measure.

2)  Drag the left handle of the wave file right up to the very
    beginning of that first beat.  The more you zoom in the more
    accurate everything will be, but don't be too silly about it.

3)  Drag the track full left, so the first beat is right at the
    very start of the window.

4)  Find the first beat of the seond measure.

5)  Enter numbers in the BPM box until the gray line lines up with
    the very start of the second measure.  That's the BPM for this
    measure, and what we'll use as the starting point.

Now turn the grid on, slide the track over at least two measures,
maybe three (so you have space for the pickup (if any) and to put
in a count-in click, for recording more instruments).  Drag that
wave file's start handle back to the left, enough so you can hear
any pickup before the first beat.

If you used a BPM-finder, just set the initial BPM to that value,
and drag the track so that a grid line lands just before the start
of the first beat of the first measure.

At the start of the first measure, right click on the timeline
itself, just to the right of a measure marker, and choose "add
BPM change".  Edit the "time" field so that it beat 1, zero ticks
(e.g., 3:1:0 if we're at the third measure in the timeline).  This
sort of "anchors" that spot in place.

Now repeat these steps, proceeding from left to right, through the
whole song.

1)  Scan to the right.  Do the measure markers seem to line up?
    If so, keep going until you get to one that doesn't line up
    well.  Set a BPM change at the last measure that IS lined up
    well (as described above).  Don't change the BPM value, just
    create the BPM change thingy.

2)  Click on the timeline and set the time cursor to the RIGHT of
    the marker you just made.

3)  While looking at the spot where the grid doesn't line up, adjust
    the BPM in the little BPM window above the timeline until it
    does line up.  (Make the BPM smaller to shift the gridline
    right, larger to shift it left.)

4)  Repeat steps 1 through 3 until you reach the end of the song,
    or you go crazy, or until your honey shouts "Don't you ever do
    anything but sit there at that stupid computer?!!"

Um ... Much easier to do a click track first!