your first thought was right, the last 3 days of the month rule is the end, that way there is little to no break in your pay. (VA benefits are paid from the first day of the month), I was just pointing out that before you start terminal/permissive, you can elect to final out at like 8am and they still do not count that day as leave, so even if you get shorted by half a day of leave, you can " get it back" by final outing early and being done.
Once you receive your ratings and you "accept" part of the paperwork is to select your final day, or you can leave that one blank and just fill in how much leave you have/want to take and permissive and let afpc do the calculation for you and select the day. in my case I have/had 62 days of leave the day I turned in my paperwork, will gain 7.5 by my final date and after you add in the 20 days permissive if retiring that left me with 29.5 days I "had" to take, and 60 days of leave I can either use or sell. So I elected to go for the end of may (89 days technically), so I could use half and sell half. I just hope my orders come in time for that to work out, as I really wanna be on terminal by april 7th.