... I understand that you want to know how much length you will loose when you bury the tail for your eye splcies but there's another way. When splicing several lines with the same diameter, the same length of bury and eye size on each splice you can measure the first splice and use that length loss for each splice thereafter. So, before you do your first splice mark your line 3 or 4 feet from the bitter end and when you finish your splice measure from the eye up to the mark on your line. That's the length it takes to make each splice.

That length will vary if you change the length of your bury or diameter of line so just keep everything the same and you'll be fine. Constructional stretch is another thing all together. You will see that your line will stretch after being loaded for some time. How much is a big question because there are so many vaiables in rope manufacturing like spool tension (a major factor in constructional stretch). You can not count on that being exact every time.

BTW, a Brummel splice is a locking splice which will reduce the line's overall breaking strength much like a knot. Is there any reason why you wouldn't do a standard 12-strand splice? I only do a brummel if I don't have enough length in the line to bury 3 fids.