Well, first, I would prefer to use EGTA rather than EDTA, because it chelates more specifically Ca2+, while EDTA equally binds Mg2+. Second, I do not think that salt of Ca is so important, but other organic anions present in a pipette solution may also dind Ca. For example, gluconate which is sometimes used as a major anion in pipette solutions definitely is rather effective Ca chelator. Another important Ca-binding agent is ATP.