Reference category can be overcome by a statistical approach called Quasi-variance. When estimating the effects of a categorical explanatory variable within a statistical model, quasi-variance is used. They are the approximations of variances. In the below Quasi variance formula just substitute the values of mean and number of data in the set and do the operations demanded by the formula to determine the Quasi variance. Mean should be calculated on your own for using in the formula.
You can also use the related calculator given below which is designed based on the Quasi variance formula to verify your results.