A doughnut spare is likely to have way more than 2% difference, so it shouldn't be a problem for a 19" tire either. Just take it easy. (I always put the doughnut spare on the non-drive axle as well, unless I'm literally only going a couple of blocks).
Have you thought about doing a blog on tires or other topics here on TMC? Your expertise is greatly appreciated and think a blog format where you could compose something for the common questions you get and solicit comments below the post would be great as well.