This is the corrected information for the OP where I missed the 1985 playoff game. If the Mods want to edit the OP they can and delete this post. Up to them.
21 times the Cowboys have started a season at least 4-1. Here are the results of those seasons.
1966, started 4-0-1, played in the NFL Championship for the right to go to the Super Bowl. 0-1
1967, started 4-1, played in the NFL Championship for the right to go to the Super Bowl. 1-1
1968, started 6-0, played in the Conference Championship. 0-1
1969, started 6-0, played in the Conference Championship. 0-1
1972, started 4-1, played in the Conference Championship. 1-1
1975, started 4-1, played in the Super Bowl.2-1
1976, started 5-0, played in the Divisional Playoffs. 0-1
1977, started 8-0, won the Super Bowl. 3-0
1979, started 4-1, played in the Divisional Playoffs. 0-1
1980, started 4-1, played in the Conference Championship. 2-1
1981, started 4-1, played in the Conference Championship. 1-1
1982, started 4-1, played in the Conference Championship. 2-1
1983, started 7-0, played in the Wildcard round. 0-1
1984, started 4-1, missed the playoffs.
1985, started 4-1, played in the Divisional round. 0-1
1992, started 4-1, won the Super Bowl. 3-0
1994, started 4-1, played in the Conference Championship. 2-1
1995, started 4-1, won the Super Bowl. 3-0
2003, started 4-1, played in the Wildcard round. 0-1
2007, started 5-0, played in the Divisional round. 0-1
2008, started 4-1, missed the playoffs.
So, out of 21 seasons, 19 times the Cowboys have made it to the post season. So that is an 90% chance of going to the post season. 3 times they won the Super Bowl and 9 times they played for the right to go to the Super Bowl. In these 19 seasons the Cowboys have a post season record of 20-16. These 20 wins represent 61% of the total post season wins we have in our History.