I have determined my TCs using several nights of data and have also calculated the s.d. of each TC. In most cases, the errors are between 0.01 and 0.02 for each filter. The question is how to include the s.d. for the TCs in my total error for each variable star measurement. All other sources of error (s.d. of the variable from several images, SNR of the variable, comp star errors, etc.) are added in quadrature to obtain the total error. However, adding the TC errors in quadrature with the other errors doesn't seem to be correct. Since applying the TCs to the instrumental mags has only a small effect on the data, the additional error seems to be a fraction of the error of the TC. For example, a TC with a s.d. of 0.02 mag adds about 0.005 to 0.01 mag of error to the final variable star measurement. What is the proper way to calculate the contribution of the TC error to the total error?