hi there,
i'm so sorry to hear about your kitty- its just not fair!

my boy, ozzy has been struggling for a while now- i know what you're going through.
as far as how much he gets tapped for- ozzy seems to get heavy breathing at about 100cc's- but sometimes its hard to tell.
before my vet got their ultrasound machine, they would just do their best...when ozz was first diagnosed, they'd take x-rays before they'd tap him...then they'd just go in blind, and try from each side...now my vet can look and see where the fluid is with the ultrasound, and watch it get sucked out.
i highly recommend an initial ultrasound, if you haven't already had it done. it can tell a lot of things.
my personal opinion, and i'm no vet- if your kitty is still breathing too hard after being tapped, she needs to get tapped for more- i think bassett gets a mild sedative for getting tapped, luckily for me ozzy tolerates it really well.
have you gotten xrays lately? or an ultrasound?

one thing i know, is that you'll definately get support here!!
so keep in touch, and keep us updated...know that we understand how hard this is.
take care
ozzlou