less than 50 calories is ok
No it's not. It defeats the purpose.
Fasting is 0 calories, period.
The purpose of fasting is to make your body burn fat for energy. You eating ANY calories is giving your body energy and your body responds like "oh I don't have to go into my fat stash for energy, food is coming" and it stops burning fat.
Flat out, you can drink plain water, plain green tea, or plain black coffee. Pretty much anything else, including stevia sweetened drinks is probably raising your insulin levels, which is again, counter-productive. It breaks your fast.
The point is to keep your insulin levels normal so that your body burns fat for energy, instead of going off of spikes of insulin. If your insulin levels are high, it's hard for your body to burn fat.