> I just don’t know how to adjust this value so by leaving it at the average, I can avoid introducing more assumptions.

This is a fallacy of assuming the midpoint equals the centroid. The correct way to handle absence of key information is to widen the error interval, not to assign zero weight to unknown factors. Either those factors have some partially known weight limit and you can estimate a range for the average, or those factors have completely unknown weight and you cannot make any rational estimate of the average.