No, or at least not how i read your reply. You need to do your leave form with your S1/HR section now. Then provide it to whoever on your post will process your orders, because they will set your retirement date based off the forms you submit.
It's an UP TO 90 days, not a guaranteed 90 days. If you don't have the leave and or pTDY, you wont get those days. If you have 0 leave and 0 pTDY (because you didn't request, or just don't have) you'll get between 5-10 days only to out-process and be fully out of service. If you have 30 days leave, you'll be give a few days to out-process and such, 30 days and that will set your retirement date.
Just to be clear I'll try a different way to explain. You sign 199. Army processes it, sends it to bureaucratic ether. One day you'll get orders. The number of days between the day you get your orders and the day you are dropped form Army Rolls and become a Veteran in the yes of the VA. Is not to exceed 90 days. The final date is based on the amount of pTDY and leave you request. If you don't request it, you don't get it. So who ever is processing your orders back in the bureaucratic ether, needs your DA-31 to know how many days to set on your orders when they cut them.
Think of it like taking leave in route and your loosing S1 making the DA 31 for the post PSB to determine your reporting date that is far enough out to allow your requested leave. Only this time you are reporting to Veteran-hood. No leave form to orders generator, no leave days, short report window.
Sorry if I beat a dead horse, but at least it makes glue and glue makes things stick.