You are apparently referring to Xavier Jubier’s excellent interactive map, which was linked to in the OP this other thread, not my OP above in this thread.
Anyway, your math is off, and is not necessary regardless. The interactive map shows the eclipse duration for any location you pick along with the start and stop times. It is not necessary to do any subtractions.
Per the interactive map, the duration of totality where it enters the Atlantic in New Brunswick is 195 sec (3m 15s). This is certainly not the longest duration over land, nor is it in the central U.S. Instead, the longest duration is in Mexico (highlighted as “Eclipse City” on Jubier’s map.
ETA: Thank you for reminding me of the existence of this map. That other thread has been going for so long I completely forgot about it. It’s a lot easier to use than that cumbersome eclipse calculator I linked to in my last post.