sorry, I’m just a little confused because you say you needed to reduce the crotch length but then in the next sentence you said you added to it. I’m going to assume that you meant you subtracted and not added. Because the different is so great, if it were me, I would use the method shown in the video to subtract 1.5″ from the front and back so it’s the same, that gives you 3″ and then with the new back pattern, you can subtract another 2″ but do it from the top of the crotch curve, where it’s more straight and then blend the line from the waist side seam to the new point on the crotch line. I think it’ll work out a little better but it is a great deal that you’re removing from the pattern so I honestly don’t know what the pattern will look like with such a huge alteration. I’d definitely make a muslin if I were you to test it out.