Taking into account that we usually obtain much more measured reflections than required to solve the corresponding system of equations, it is reasonable to apply a least squares refinement to minimize the random errors. Partial differentiation of the sum of residuals squared leads to a linear equation system which can be solved in cell constants and D by any standard method. Correction of absorption errors could be included using a second term E for the absorption correction. Another way is to introduce an angular dependent correction for the error in theta due to absorption. In most cases the resulting differences between the corresponding refined cell constants lie within the range of error.
Wild differences require investigation.
In case angle dependent drift remains fairly constant, it corresponds to a zero angle offset for a diffractometer. Please note that it is in principle incorrect to use a drift correction while employing an internal standard. In this case you can abuse the drift constant as a fudge parameter to minimize into a better but physically not meaningful residual's minimum. Such a low drift value usually does not change the cell constants within the standard deviations. There are some remarks necessary concerning the actual accuracy of cell constants.