I added 1/3 C more Cake mix for each box I used. It worked great. Cake came out great. When I did not add the extra mix the cakes came out sticky on top. Now they work just great with the extra mix.

Thanks for letting us know what works.
Yeah, that's what I've been advising people but many don't want to deal with an extrar particial box of mix All they have to do is fz it until the next time they bake.

Thanks for the info. Yes, we can even empty a box into a plastic container also. Just get you one of each flavor you use. No problem. Thanks again.

What brand of cake mix do you use? 

I prefer using Betty Crocker.   I've been making the *original* WASC recipe using the smaller sized box mix for awhile now and it comes out perfect in my 9x12x2 Magic Line pan with NO changes.

AUgh... I had a bad experience with Betty Crocker mixes once and was so mad I swore I would never use them again!

Does this work for all cake mixes? (adding the extra  1/3 cup). I always use DH. I dislike BC since they changed. Even baking BC without the WASC recipe came out horrible..But maybe using the extra mix will help.

I do it by weight, so for each mix I use, I add 3 more ounces of mix. 

I primarily use DH, so I also add a packet of instant pudding mix for each box.