Therm, ah, no. That is not correct. Straight gauge spokes are weaker than double butted spokes in practice and theory. It's not that the flange and spoke end are "thicker" as you say for strength. It's that the butt or thinner section of the middle of the spoke takes up the tension that would normally would be put on the threads/nipple and angled flange. This is why you will see a straight gauge spoke fail more often than double butted. The fatigue of the spoke does not take place in the center of the spoke. It is always transferred to the elbow and threads/nipples. This is where you normally see spokes fail. Straight gauge put more/transfer fatigue on/onto those two areas. Period. Double butted pull some of that fatigue off the same two areas. This is just a fact.
I was giving an optimal as far as length on both sides of the butt. Yes it can be off some but you cannot cut down the straight gauge portion of the end of the spoke too much because you begin to lose the whole purpose of a double butted spoke. If you're going to do that, then just replace with a straight gauge spoke.