Here is a link and the info. It appears there is no set formula, but the common sense is that CBS/FOX will veto the NFL trying to move an otherwise good game off its network to SNF.
Flexible Scheduling
Allows teams to play their way onto the Sunday night schedule
Last season, the NFL implemented for the first time in its history a primetime "flexible scheduling" element on Sundays in Weeks 10-15 and in Week 17.
Flexible scheduling ensured quality matchups on Sunday night in those weeks and gave surprise teams a chance to play their way onto primetime.
The 2007 NFL schedule will again utilize flexible scheduling in Weeks 11-17. The NBC Sunday night time slot for "flex" weeks will list the game tentatively scheduled for Sunday night.
Only Sunday afternoon games are eligible to be moved to Sunday night, in which case the tentatively scheduled Sunday night game will be moved to an afternoon start time. Flexible scheduling will not be applied to games airing on Thursday, Saturday or Monday nights.
Just as the six major college football conferences have done for many years, the NFL will have the flexibility to move the start times of games on Sundays, using a 12-day notice format.
For example, a game scheduled for a Sunday could move from a 1:00 p.m. ET kickoff to an 8:15 p.m. start, but the change would be made and announced no later than the prior Tuesday, 12 days prior.
The NFL has commonly moved games between 1:00 p.m. ET and 4:15 p.m. ET on Sunday afternoons, and the NFL now employs flexible scheduling to include one of its primetime packages of games - on Sunday evenings.
Sunday afternoon games, as in the past, can still be moved between 1:00 and 4:05 or 4:15 p.m. ET.
In Week 17, in order to ensure a Sunday night game with playoff implications, the decision to move the start time may be made on six days notice.
NFL Flexible Scheduling 101
Begins Sunday of Week 11
In effect during Weeks 11-17
Only Sunday afternoon games are subject to being moved into the Sunday night window.
The game that has been tentatively scheduled for Sunday night during flex weeks will be listed at 8:15 p.m. ET.
The majority of games on Sundays will be listed at 1:00 p.m. ET during flex weeks except for games played in Pacific or Mountain Time zones which will be listed at 4:05 or 4:15 p.m. ET.
No impact on Thursday, Saturday or Monday night games.
The NFL will decide (after consultation with CBS, FOX, NBC) and announce as early as possible the game being played at 8:15 p.m. ET. The announcement will come no later than 12 days prior to the game. The NFL may also announce games moving to 4:05 p.m. ET and 4:15 p.m. ET.
Week 17 start time changes could be decided on 6 days notice to ensure a game with playoff implications.
The NBC Sunday night time slot in "flex" weeks will list the game that has been tentatively scheduled for Sunday night.
Fans and ticket holders must be aware that NFL games in flex weeks are subject to change 12 days in advance (6 days in Week 17) and should plan accordingly.
NFL schedules all games.
Teams will be informed as soon as they are no longer under consideration or eligible for a move to Sunday night.