I take it you mean caustic soda, AKA sodium hydroxide? Baking soda isn't alkaline enough to do the job.
Yes caustic soda is pretty poisonous ( though it does get used as an ingredient in pretzels ... the dose makes the poison in this case ) and, well, caustic. Which is why you should be being very careful to avoid splashing the solution around and taking care not to pipette any of the bark soup up with your NPS. If you get any on your skin then wash it off immediately under the tap, you'll notice a soapy feel, this is the caustic soda making soap out of the fats in your skin!
The quantity to be used is primarily related to the amount of water you are using, not the amount of bark. You need to achieve a ph of greater than 12.5 in order for the freebasing to occur.
20g in a litre should be fine, as long as you haven't used lots of acid in a previous stage that will need to be neutralised to achieve the required ph level.
Extra NaOH will act to help prevent emulsions and perhaps break down plant matter to liberate more DMT. Don't worry about having used 50g instead of 20g, this time. It won't hurt the final product.
To be extra safe you should water wash the NPS as it is pulled off the soup, this removes any suspended caustic soda solution micro droplets from the NPS.
To water wash, simply squirt the NPS into a jar containing a little water as you pull it off of the soup, then you can wait for the NPS to settle into a nice layer and pipette that off the wash water into your freeze precipitation dish. Adding a pinch of sodium carbonate to the wash water will help prevent any slight acidity in the water from turning some of your freebase back into a soluble salt again and thus being removed from the NPS.
See here for lots of discussion on this:
The Base in Your Product