Typically you'd want to use a metric that is expressly a percentage, such as MAPE or R^2.

That said there are a few ways to turn your RMSE into a percentage, the difference being what denominator you use. In your case it's important to find out what denominator was being used in the paper you are comparing to, as using different denominators is not comparable. The popular denominators to use are:
- Mean
- Range
- Standard deviation