Yes, if you aren't very good 'eyeballing it'....then make your plunge cut starting about 1/2 way up from the bottom of the trunk. Keep your bar buried (let the saw do the work) and saw downward until you come out the bottom on one side. Then take a piece of string (or small rope, small enough to fit in the saw kerf) and wrap it around the log. You'll be able to see if the rope/string is fairly straight and where the two ends overlap...make a mark. Use that to line up your second plunge cut.One last question before I go after the root ball end. Is there a handy way to ensure I'm aiming for the same point when plunge cutting from each side? Or just spitball it?
Bore in and cut down that side until you come out the bottom. Then move to the top. Once you've made enough cut to get a wedge in your saw kerf...you can continue cutting down toward your bottom cuts. Keep a couple of wedges in your top cut to prevent the trunk from closing up on your bar. Be ready to move back when the two sides separate. The root ball shouldn't fall back into the hole doesn't look like, but do expect some movement.
Once the entire trunk is on the ground (no air underneath it) you can switch back to cutting solely from the top down and again use your wedges to keep your kerf open (avoid pinching your bar).
Bet you are tired of working on it by now? Why can't these things happen in October when the weather is nice in Kansas.
NOTE: Remember to roll your tip into the log and DON'T stand directly behind the saw until you've got it started well and begin your push straight in. Bore/Plunge cuts are more often horizontal (felling trees) where you are naturally more out of the way. So be careful not to get any kickback.
Edit: Sent you a PM, an easier way to line up your plunge cuts, but not for the inexperienced, so not posted here.