How to determine amount of CO₂ in soda water
First of all, after degasing by heating you have to be sure that no $\ce{CaCO_3}$ is formed, so you can minimise this problem by transfering the water in another flask.


More accurate would be to start directly with demineralised water which you can buy cheap at filling stations.


Secondly, you shouldn't wait to much while titrating, otherwise the water is taking up $\ce{CO_2}$ from the air.


You should use a bulb which doesn't contain much air (which contains $\ce{CO_2}$ and can interact as well. You can use a round flask and directly connect it with a stopper.


Third, Phenolphtalein is a good indicator because you titrate a weak acid with a strong base.


Make sure, you don't use too much Phenolphtalein.


Last but not least be sure that the $\ce{CO_2}$ from flask doesn't go away in the air or into the NaoH of the burette.


To cool will help, because $\ce{CO_2}$ is more soluble in cold water.


If you want to be a super-purist, do a potentiometric measurement and plot the point of change using a simple voltmeter.