My estimate is 21,500 crossovers. We know only 6,000 voted in the GOP primary, and I estimate that about 25% of the voters are behavioral Republicans. If it is 29% as Barone said based on the Bush vote (I think that is too high, because some white Democrats in the South voted for Bush), then the number of crossovers is higher.
It sounds like you've done a deeper analysis than me. I was just taking the number of Majette voters in the northern part of the district, and discounting it for a few Democrats, because the number of 'Pubbie crossovers up here was HUGE. Then picking a number out of the air for 'Pubbie crossovers in the rest of the county. My upper end and your number are pretty close, for what are really both non-scientific guesstimates.
BTW, the Majette:McKinney totals were 12,039:601 (20:1!!!) in the 16 Dunwoody / North DeKalb precincts. There were only 1872 votes cast in the 'Pubbie primary for State Rep (Millar vs. Davis) in those same precincts. This area is heavily Republican in a national election (heck, we used to be in Newt's old GA 6th before '92), so you can see the crossovers were substantial up here.