So, just because a state has voted Republican in recent elections, means that they should get a 20+% advantage there?
But, to humor you, lets take a look at some past election results. The number is advantage in percentage points the Republican candidate had over the Dem candidate:
2004 - 22.87 (Governor of State was running)
2000 - 21.32 (Governor of State was running)
1996 - 4.93
1992 - 3.48
1988 - 7.72
1984 - 27.5
1980 - 9.74
Yeah, that 15-20%+ is super realistic.