Yes, that's correct. The law actually specifies that the deduction is calculated that way. It's reduced by 6% of MAGI in excess of $75,000 ($150,000 for MFJ). The phaseout range that I stated is the result of that calculation. The law specifies the 6%, not the beginning and end of the range.
I would just point out that you are assuming that both spouses on the joint return are eligible for the deduction (both 65 or older and have a Social Security number). The deduction is calculated separately for each spouse. If only one is eligible, that one gets $3,900 in your example, and the other gets no additional deduction. The "potential" deduction is $6,000, not $12,000.
