I don't think there is anything wrong with a longer splice that I can think of other than more of the line is fat. Brion talks about using rope diameters instead of fid lengths so if it were me even though I was actually using the 7/16 fid I would measure with the 3/8 one. It sounded to me like the larger problem was that your splice was twice as long as it should be and I assumed that was because the fid and the instructions didn't match. You might want to read this:
http://www.briontoss.com/education/BTR_book.pdf
I have both the Samson fids and Brian's splicing want and I prefer the splicing wand.
I think he is using 24 diameters which is 240mm or 9.5 inches. Just Google "convert 240mm to inches" for your future reference. Check the link above to make sure where that is measured to and if it applies to your core dependent line or not.
Allen