Explanations and arguments have different goals, but they have the same logical structure, and the explanandum should follow from the premises. A good explanation will show us that the explanandum is not mysterious, but is something we might have expected all along, something we might have predicted or inferred from the facts mentioned in the hypothesis if we had known about them.
The best way to measure strength in this context is the general-purpose method we learned in Chapter 5. We try to estimate the size of the gap between hypothesis and explanandum, the amount of "free play" or slippage between them. We do this by finding the implicit assumption that would be necessary to close the gap.