Note: this feature is not covered by the "Start Printing at xx mm" functionality.
If I wish to fine tune the settings for a particular portion of the model, it would be useful to say: Start printing at layer # x and stop printing at layer y".
For example, in debugging a church steeple I want to test certain settings for just the steeple portion without printing the entire church. I tried using the "Start printing at..." feature, but that starts the steeple printing in mid-air. Also, the units should be the actual layer number, not a height in mm. Even in the context of the current functionality, specifying the height in mm seems odd and inaccurate when the operational context is always about specific layers.
So to summarize: Select a section of the model, have it drop that section to the printer bed level, print just that portion.