Heating elements will have different surface temperatures under varying air velocities.  The maximum allowable surface temperature of the heating elements will be under still air conditions. As air velocities increase around H.E.'s, their surface temperatures will decrease.  Your best bet is to contact manufacturers of these H.E.'s and ask for a temperature profile vs. air velocity.  The results will based on a lineal measurement.  Then you'll need to determine the smallest  design surface temperature which these heaters are set to operate so that you can determine the CFM based on the total number of H.E.'s and their total length.
Two problems with your formula.  One is the delta and the next is the BTU/HR or in your case BTU.  Both these values are used by the manufacturer and you have now idea what number they are using.  
BTU/HR is determined by the allowable surface temperature as mentioned above.  The delta T is based on recomendation by HVAC design guidelines.