How can I determine the best trace width for my pcb?
On the website you've linked, there's the in depth-description on how it calculates the width. The more information you put in, the more reliable is the calculate.

To avoid any problem I suggest to oversize your max current.

You wrote you've calculated for 70A. If the current exceeds that value it's possible that the trace breaks. Of course, it also depends on the time that the current exceeds that value so, first of all, I would check the max critic current your board must bear and calculate the width with that value.