TV stations are not protected by contour analysis like FMs are, they are protected by a statistical analysis of coverage and interference within their protected contours.
https://transition.fcc.gov/bureaus/oet/ ... /oet69.pdf
That is the FCC document explaining how it works in depth, but a short version:
Each station's coverage area inside its contour is broken up into "cells" that are individually evaluated for coverage and interference using Longley-Rice. If the population within the cells that receive interference adds up to more than 0.5% of the total population that would receive the protected station without that proposed station (for full-power and Class A; it's 2% for LPTV), then it is not acceptable to the FCC. Stations can opt to accept interference above those levels if they wish, though it would be unusual for a full-power to agree to accept interference from an LPTV station it does not also own.
And yes, the D/U ratio is 15 dB, but there's an adjustment factor in weak signal areas, among other things, that come into play.
To a few other comments that caught my eye:
RabbitEars shows 41* dBu contours for every UHF station, but the FCC only protects LPTV and Class A stations to 51* dBu. Full powers are protected to 41* dBu. (* It's 41 or 51 dBu with a frequency-specific adjustment factor; see OET Bulletin 69 linked above. VHF contours also vary between full-power and Class A/LPTV, but do not have a frequency-specific adjustment factor.)
The top available channel is 36. During the repacking process, no station in Canada was assigned to channel 36, but that channel was assigned to US stations.
WPVI ultimately chose channel 6 after fighting for a UHF channel, losing that fight, and being unable to find another channel in UHF or high-VHF. I'm further not sure why the station would be called "WTVI", as WTVI is a PBS station in North Carolina.