No worries. Your instincts are good.
I gauge the length of a replacement by looking at where the seams will land. I try not to make the piece unnecessarily long, but I try to span at least three ribs while avoiding adjacent seams.
If you are replacing a piece that has a lot of shape to it, you can get it to fit by using a wet cloth and a steam iron. The heat/steam helps to shape the plank. Take care to get your gaps right before you tack it in. If the edges end up a bit proud (as they will on the curved sections) use your micro plane to remove the high spots.
If you are having trouble with planking splitting, consider predrilling with a small brad or whatever you have on hand.