Hi Elizabeth
As you can read I had a bit of trouble with that same issue back when I was new to FTS and trying to adjust to a different way of baking. Like Doug said be sure to taste that batter before and sweeten accordingly. Keep in mind though that some sweeteners will lose their sweetness when heated. The xylitol and FTS zero doesn't seem to do that. I make this recipe all the time now with just a few personal adjustments for me. I sweeten mine to the equivalent of 3/4 cup and add in some sugar free chocolate chips.....if no chips I might even sweeten to 1 cup.
Also I find that if I mix the sweeteners I like the finished product better. So I use half xylitol and half zero. If I'm using erythritol in place of the xylitol (xylitol is sweeter) I increase the zero a bit. So if using the xylitol mix, if it calls for 1 cup sweetness I will use 3/4 cup xylitol and 1/4 cup (12 drops) of zero.
Personally I love the way flax blends with chocolate but prefer to mix my flours with almond meal or use unblanched almond meal by itself.
Here is a trick I learned that might help you. Agreeably most items are much better when baked in the oven, but if you put a couple of Tbs of the finished batter in a microwave safe dish (I use the small silicone muffin cups or small plastic bowl) and micro for a minute. You will be able to taste how sweet it will be after it is cooked without wasting your ingredients
Always start with the least amount of sweetness and add in from there. Keep in mind it's so easy to go overboard with the zero if you aren't careful
Don't ask me how I know this one Hee hee
I hope this helps