You have left time out of the equation - it should be:

Note that I changed your "r" to .

The rest looks fine. However, be aware that since air drag is a function of velocity, by using v_o^2 in the above you are making just an estimate of the actual drag that occurs as velocity changes from v_0 to v_1. Consequently the values for the constant that you calcuate will change with each set of data. One suggestion: instead of using v_0 in the drag formula use the average of v_0 and v_1. Also use small values of t so that the difference between v_0 and v_1 is minimized.