to establish by calculation
or to give an opinion.
"I reckon if we leave at 5:00 in the morning we will be at the airport by 7:00" = your opinion based on your calculation and/or experience of the traffic
"it is reckoned that the last polar bear will be seen in the arctic circle in the year 2025" = a calculation based on scientific measurements.
"give me a reckoning for arrival at the pot of Rotterdam" = asking for somebody to calculate what time a/the ship will arrive in Rotterdam
"what is you reckoning for arrival into Rotterdam" = either a request for a calculation or a request for an opinion or a mixture of both based on reckoning and experience (opinion)
you can see that one often has a bearing on the other.
It also has the meanings of believe something to be true, or to expect something to happen
see here for more details. https://www.collinsdictionary.com/dictionary/english/reckon