I read the write up on audipages.com and the procedure looks fairly simple. Although, I worry about getting the fluid level correct without VAG. How important is it to do the VAG test after filling?

I haven't yet, but I'm about to. I have the OBD adapter to use VAG-COM, but I haven't got my Pocket PC to work with it yet. If I can't, I'll get an inexpensive laptop off of eBay to use the Ross Tech software before I do the fluid change.
I suppose without the Vag software, and if it's the first fluid change, just drain, measure what comes out, and replace the same amount. The hardest part is getting the new fluid in if you dont have the factory filling tool.

Assuming the present fluid level is correct... If I precisely measure what comes out and refill with the same amount, how could I go wrong?

But what do I know? I'm not an Audi technician and this is just my theory. Somebody please chime in and let me know if I'm mistaken.

to the temp of the ATF. With the car idling in "P", all electrical accessories off, the fluid level is correct when it just flows out of the fill port w/ temp 35 - 40 degrees C. The car must be level when you check the level. VAG-COM or other scan tools (e.g., VAG 1551/1552) are used to indicate the ATF temp only, nothing else.

The original VW Audi tool don't have pump. It has a valve to open the fluid flow and the you just let air to vent into the bottle. It will fill up your tranny the same way as you would pump it. I never Vag-com my A8. The filling procedure will take you at least 5 minutes, the temperature will be just fine when you done.

Checking and correcting ATF level on transmission without ATF filler tube (vehicles 6/94):

The ATF level is checked at the ATF check-plug hole.

The ATF level is correct if fluid slightly runs out at the ATF check-plug hole with ATF temperature between 35C and 45C) (caused by the increase in the fluid level during warm-up).

Older vehicles with automatic transmission may also have been retrofitted to the filler system without the filler tube described below.

1) For hot climates 50C

WARNING!
Wear safety glasses.

- Attach supply reservoir of VAG1924 ATF filler tool at the highest possible point on vehicle.
- Place catch basin under transmission.
- After ATF temperature of 35C is reached, remove ATF check-plug and if necessary drain excess fluid.
If ATF leaks out of ATF check-plug hole before ATF has reached 40C, ATF level is OK.

Note:

The ATF check-plug must be reinstalled when ATF temperature reaches 45C).

1) For hot climates 50C

If at 40C, ATF does not leak out at ATF check-plug hole, top off ATF using filler hook from VAG1924 ATF filler tool until ATF runs out from ATF check-plug hole.

is that all of this must be done with the car level. Okay, raise your hand if you are able to do any work under your car while it's sitting on the ground. Okay, now how many have a lift bay in their garage? So..If you didn't raise your hand to either one, you best have it done by a shop that can do it right.
(or buy 4 jack stands, buy/make a fluid filler tool-VAG 1924 or equiv, VAG-COM setup, which is already more than the trans service would cost to begin with...)