Hello, all.  First time poster.  (Gulp)

I doubled a cake recipe in order to go from an 8" square pan to a 9 x 13 pan.  The result was light and tender but very dry. 

I doubt that it's overbaked, because the cake had JUST BARELY started to pull away from the pan sides and I took it out of the oven JUST WHEN the center sprung back when poked.  It was just starting to turn golden.

My guess at a solution?   Remove one egg white, reduce baking powder to 3 tsp and increase sugar to 1 1/2 cups

Here's the doubled recipe that I used:

1 cup butter-flavoured margarine, frozen  (7.3 oz)
1 1/3 cup sugar
4 eggs
2 tsp pure vanilla extract
1 cup + 3 Tbsp milk
2 1/6 cups Cake & Pastry Flour (13.3 oz), sifted
4 tsp baking powder
1/2 tsp salt

Grease  a 9 x 13 non-stick baking pan.  Line bottom only with parchment; grease parchment.  Place 4 greased flower nails in a line down the center poking through parchment,   Preheat oven to 335° F.
In a small bowl, whisk flour, baking powder and salt and set aside.  
Place frozen margarine and sugar in bowl of stand mixer fitted with paddle attachment.  Beat until fluffy.  Add eggs one at a time and beat to combine.  Whisk vanilla into milk.  Without over-beating, add dry ingredients and milk in three additions folding the last dry addition by hand.  Mix just until flour disappears.  Do not overmix.  Scrape down sides of bowl as necessary.
4. Pour batter into prepared 9 x 13 pan.  Smooth roughly with offset spatula.  Bake at 335° F. for 22 - 25 minutes,  rotating pan half-way through.  Cool 10 minutes on a wire rack, then de-pan.

Suggestions welcome!

i suggest you get a recipe that works in the first place like from a famous cake decorator -- like whosoever you want -- i am not one to mess around with trying to fix a bad recipe  i've been baking forever and i don't know how to fix that -- and i sure do not want to waste ingredients trying -- but i will say this -- i firmly believe in mixing the cake batter for two minutes after it is all combined -- no more than 4-5 speed on a ka type stand mixer --