Segment customers by a driver that meaningfully changes demand rather than relying on one average.

In concentrated B2B markets, customer size, usage intensity, location count, capacity, or industry can produce very different purchase volumes.

You could estimate large, medium, and small customer groups separately, calculate typical annual demand for each, and then sum the segments.

Keep the segmentation simple enough to calculate and explain why it captures the concentration. market sizing guide.

This is way better than using one fake "average company" when the top customers are huge.