Is there a specific formula or ratio that I should follow when making additions to bx cake mixes or dry to liquid? Liking adding puddings/purees or yogurt then adding liquids like juices/flavors??? When is it too much liquid or too dry not enough liquid? I'm just never sure how much I can add per box???

Sorry!!! Is there some rule of thumb to follow when changing up a recipe???

Not sure about a formula but most extender recipes call for 1 1/3 cps of liquid(per cake mix), 1cp flour, 1 cp sugar, 1 cp sour cream or yogurt, 3 or 4 eggs. It's also fine to add a small pkg of pudding to that mix.
It depends too on the cake you are making. Sometimes if the liquid you are adding is sweetened you may want to add a little less sugar.
Hope that helps

I would either stick to the amounts on the box or near them, the amounts in a doctored recipe, or the amounts in an extender recipe.

A doctored recipe is usually the items called for on the box, plus an extra egg and a thing of dry pudding.

You can replace the water called for with milk, buttermilk, juices or sub in some alcohol.

With the extender recipe you can change up the water, and the flavors of the yogurts. I would though, stick to about the same measurements for the liquids vs dry.

For a fruit puree, I'd probably sub that in for some of the water, since you are adding in a liquid type of ingredient.

Thanks sooooo much much! I was thinking along those lines but just wasn't sure if you added additional or just sub it out...I guess it all depends what I am adding. The yogurt / fruit subs is where I was having the most trouble, wasn't sure if solid or liquid....lol... sorry... I'm a recipe girl but wants to try some new things...I'll just keep trying!!!

.......The yogurt / fruit subs is where I was having the most trouble......

Just remember that fruit, for the most part, contains a high amount of water so when adding fresh things like apple, peaches, carrots you should probably reduce the liquid called for about 1/4 cup - but that will depend on the recipe used in the first place.
Adding yogurt or sour cream will make a cake recipe more dense - not as 'fluffy' as 'normal'.