I find if there is too much stevia in a recipe, it tastes very bitter. So I prefer to err on the side of too little.

So a general rule of thumb I use is 1 tsp. stevia extract = 1 cup sugar.

I use this equivalency to replace the sugar listed in a regular recipe. That way it doesn't come out too bitter.

If I want it sweeter, then the next time I make it, I add a tiny bit more stevia to see how it comes out, or--and this is what I have been doing recently--I add erythritol.

To Godspeed, to answer your question about 1 TBSP of Splenda: with the above formula. that means I'd add 1/16 tsp. of stevia (16 tablespoons in a cup). With that small a recipe, if I wanted it sweeter next time I'd add erythritol.

So far this has worked for me!