Intracellular calcium is heavily buffered so that the concentration tends be be very low. As a consequence the reversal potential for calcium can not really be modeled as a fixed value. The intracellular calcium levels are on the order of 1000 times lower than extracellular calcium which accounts for the rather large reversal potential for calcium. The way that many researchers model calcium is through the constant field equation
as is usual. Then in the above, it is clear that V=ECa makes the current vanish so that the ``reversal potential'' is indeed the Nernst equilibrium for calcium and linearizing about this reversal potential, we get the slope
which if you check has dimensions of conductance per unit area. Thus, we can approximate (5) by
where ECa is the Nernst equilibrium potential. We will use the two interchangeably, but keep in mind that the constant field equation is more correct.