That makes sense then. Originally I assumed you had at least 7 days of vaca.

(V-days x 6 + 6CH) ÷ R-day CH value (round down) minus V-days in the month. That gives you # of days of extension available.

So, in your case 24 ÷ 4.9 = 4 - 3 = 1 day of extension available. I don't know if it's a factor as well, but remember you'll have a 48 hour buffer after the extended day (unless you waive it). So, if you have R-days starting on the first day of the month, the 3 under your vaca are gone plus the next three (1 extension plus buffer). If no R-days are close enough to the vaca to be involved, then you just knock out 4 days total with the extension.