Replacing sending unit- how low does the tank need to be???

I assume that you'd want as little fuel in the tank as possible before replacing the sending unit...but I'm in the middle of swapping out my end and right now I have totally unrestricted access to the sending unit. Were it not for the fact that I have a nearly full tank of high octane/stabilizer in there, I would have seized the opportunity already.

I don't want to syphon gasoline in my garage (ranch style house with garage downstairs) so I think I'll miss this opportunity. I'd guess that I'm ~80% full...but of course, I don't know for sure because the gauge doesn't work. :D

So for future reference, how full can a tank be to allow for pulling the sending unit?