I understand it this way:
2008 - Awarded 68,000 @ 25% (Based on 275,000)
2016 - Awarded 74,000 @ 25% (Based on 310,000)
Retro should be:
2008 - 360,000 * 25% = 90,000 (90,000 - 68,000 = 22,000)
2016 - 360,000 * 25% = 90,000 (90,000 - 74,000 = 16,000)
22,000 + 16,000 = 38,000
Ish...