Hi all, the amount you can drain from a liner pool safely depends (mostly) on how old the liner is. A newer liner (> 5 yrs) can have the water drained to ~ 2" in the sallowend and still not have problems, nor need a vac to reset the liner. Liners lose their 'stretch' over time and become more difficult to work with, however chemically abusing them will quicken the process. Another problem facing anyone trying to reset a liner in Oct, is the temp, liners are easier to work with when the weather is warm, the colder it gets the harder it is to 'play' with them.
Andy, I would recommend letting it go below the stair bottom plate, just to be sure that it's not the stairs, but no further, this would eliminate all but the MD or a tear in the liner (probably floor). For 2 -3 " / day it's gotta be something fairly large, if it's a rip or hole you may be able to see it from the deck (leaves or debris may get sucked into it thereby obscuring your view - use your brush to check any suspect areas)
Canuckpool, if the liner has enough 'flexibility' left in it, that's exactly what we do, so it can be done but in some cases, the liner still won't go back correctly (age or chem damage).
Andy, I wish you well on this and hope you can find and correct the leak! If I can offer any more advice, please feel free to ask (and if you don't, please let us know what it was and how you solved it )