We propose a general error-driven algorithm to compute form factors in complex scenes equipped with a suitable cluster hierarchy. This opens the way for the efficient approximation of form factors in a controlled manner, with guaranteed error bounds at every stage of the calculation. In particular we discuss the issues of bounding the error in the form factor approximation using average cluster transmittance, combining subcluster calculations with proper treatment of visibility correlation, and the calculation and storage of the necessary information in the hierarchy. We present results from a 2D implementation, that demonstrate the validity of the approach; the form factor approximations are effectively bounded by the user-supplied threshold.