Thank you for the opportunity to help you with your question!
% difference = (accepted value of speed of sound- experimental value)/accepted value *100
the percent difference could be find out by the above formula.
The speed of sound is influenced by several factors, including medium, density and temperature. if the percent difference is larger that means your experiment was performed in less ideal conditions and if it is less then you have provided best conditions to your experiment.
Please let me know if you need any clarification. I'm always happy to answer your questions.