1) less gradation between untoasted and burnt than say, white or wheat bread, so the same "delta dark" would results in burnt toast.
2) the toast can (if the original bread is dry and/or dense enough) become unpalatably hard during the toasting process even before turning dark enough to be considered burnt.
For my original design, you'd definitely have to set the control to only slightly darker than your rye bread, or you'd get burned rye toast.
For those of us that have discovered the joy of slightly burnt toast with Vegemite for breakfast, accompagnied by a strong coffee, the value 15 will be seeing heavy use :-)
Of course, I'm interested to hear how you're going to go about solving the non-uniformity of toasting problem - sometimes part of my toast is still white when other parts are black!
If you could make the toaster's heating coils out of a substance that increases its resistance with increased light influx, then the bright areas of the toast would increase the coil resistance nearby, thereby producing more heat locally and tending to even out the toast.
Of course, this is probably a lot of work just to make a toaster.