Brian:
That only matters if you attach the add-on valve directly to the end of the brake line and use the existing (double-flare, metric) fitting. If you mount it there, you'll need another short length of metric, double-flare brake line to finish the connection.
Because of the nature of the fitting (you need a special, preferably hydraulic, flaring tool to get that double flare) I prefer to just cut the rear brake line somewhere along where the seat is, and then do a standard (American, single or trumpet) NA flare to match the residual valve seats and threads. I also have a hand tool that can make that type of flare at home, rather than pull out the line and take it somewhere to have it done.
Depending on how you do this and the length of the valve you purchase, you can either pull a bit of brake line through on one end to remove the slack, or simply cut out a short piece (an inch or two) and put the valve where the piece was removed. You have to be careful in how much you remove - make it too short and the lines won't reach any more!
Again, I like to put them beside the seat because you've got lots of line to work with, you can hide the valve easily beside the seat track AND there's a few places you can maneuver the line into to remove the slack. Once it's installed, you never have to mess with it (unless it leaks, of course).
gn