% EDTA is pretty high. (Human) Blood is usually collected into 0.1% solution. You prepare these tubes by adding the 0.1%solution of EDTA to the tubes then letting them evaporate dry at room temp. You can use either the disodium or the dipotassium salt (dopotassium being more soluble).
20% EDTA in PBS is even higher. The 20% probably means w/v (weight by volume). So into 100ml you'd add 20mg.
I'd probably try a range down to 15 or even 0.5%. Outcome measures could either be calcium concentration (easy spectroscopy using cresolpthelein) or perhaps the clotting time (not sure about this one). EDTA inhibits the clotting be sequestering calcium. It thus inhibits all calcium-dependent enzymes not just the clotting ones. Heparin is another anticoagulant that only inhibits the clotting. I imagine it depends what systems you are looking at as to which you use. Stick to what the literature uses and if neither have been reported...try both. If you get something interesting get human samples, repeat the experiment and publish in a clinical lab journal. (Then collect Nobel Prize).