We don't use the word "reckon" as a verb much anymore. It's like something you would hear in an old Western movie with the characters using Western or southern old-time dialect. Sample dialogue: "Hey, Pa, ya reckon that the sheriff is gonna come and arrest ya?" "No, son, that darn sheriff ain't gonna arrest no one!"

In other words, don't worry about using the word "reckon" in modern urban English!

I reckon I don't know how to use it properly.

Like Margaret said it's not very common these days. Older generations used it more frequently. I could be wrong, but I think it was used more in southern dialects. Kind of like our use of the word "y'all", it's very common to hear it in the southern states in the United States, but no so much the farther north you go.

I think the best way to think of reckon is to think of it as "I guess".

I reckon I should do something.= I guess I should do something.

It would also be common to hear oughta in place of the should in the example; I reckon I oughta do something.

It's not really the correct way of speaking, but it's part of the southern/old timer dialect.

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

And what time do you reckon that to be, either by calculation referring to a clock/watch or by your opinion or belief/hope.