Currently, Henderson is p-2026. According to Item AIX F-1.0, he would be eligible for a 60% discount due to early signing (202x + 2).
Given his current rank (#1 CI, #2 MI) the highest salary attributable (as you noted) is $30/yr. Therefore, Henderson would be eligible for:
40% of $30 = $12/yr.
CI/MI Gunnar Henderson BAL $12 (2029)