He had an independent property of
about a thousand souls, to reckon in the old style

What does reckon in the old style imply here?

Question

He had an independent property of
about a thousand souls, to reckon in the old style

What does reckon in the old style imply here?

Answer

I feel sure "reckon" means "to calculate a sum," "to add up."

I don't understand the reference to "old style." Perhaps it means that there was a tradition of describing the value of a piece of land by saying how many people lived on it, rather than by calculating its value in money.