My understanding the last time i read it was each of your hpsp years was activated for each reserve year you did. Thus for my situation...i could get out at 13 yrs, pull 4 years reserve, and get 4 years credit for 21 years total but if i only did 3 reserves then i would only get 3 credit and put me at 19 instead. Basing this of someone else's experience in my current department and reading regs...please let me know if I'm wrong.
On another note, you can work for 2 pensions if you grab a reserve retirement and gain federal employment (e.g. MEDCEN contract or VA doc). Federal law allows it with a reserve retirement but not an active duty one. Combine that with better pay, fewer hours, less admin b.s., more retirement fund matching, acess to TSP and TSP catchup if you stsy in long enough and i don't see why anyone stays in 20 active for the retirement.