I've done a couple of floors using laminate. I usually measure out the width of the room, divide that by the width of the finished part of the piece of flooring (sounds like 3 1/2?). So if the room is 12' wide (144"), divide 144 / 3.5 = 41.14, you would have just over 41 pieces across the room. The goal is to have a few inches on each side of the room, no joints within a 1" or so of the walls running parallel to the flooring. So in this case rip the first row in half, and the last row should work out to be about the same distance.
Personally I'd lay a chalk line the same distance out from each end of the room (rooms in this case) where you just clear one side of the opening. Your measurement should take in to account that first row's width and however many rows in addition to clear the opening. Once you put your chalk line down you can check your measurement to see if you'll have any large gaps and adjust accordingly by adjusting the width of your first row's pieces. I'd even go so far as to chalk each room's first row based off the reference line that runs room to room.
Once you have the first rows down you could dry stack enough boards on each side to double check your layouts.
As Ed mentioned, pull the base boards to give yourself some fudge room.
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