This seems like two questions, so I will tackle only the title question.

In the upper left of the program is  seach box (spyglass icon) Enter PERCENTAGE and see if you can find "percentage of business use" in depreciation > click that. Scroll  down to Additional Information and you should find a spot to enter a percentage for business use. 

Now the 2nd question - It isn't https://www.law.cornell.edu/uscode/text/26/179