What is your preferred blood flow rate for a patient with ESKD who has an AVF but is only undergoing an ultrafiltration session?  

It really doesn't matter very much. I don't believe higher blood flows pose less risk of clotting the dialyzer. Normally, I go with around 200-250 ml/min. If there are problems with the flow or clotting, then I would try lower BF first to see if it works better (mainly trial and error).