R square value shows tedency and is very easy calculated in MS Excel using command "=correl(array1[Ax:Ax+n];array2[Bx:Bx+n])^2" and it is calculated by formula
Root mean square error shows difference between two given data arrays. It is calculated by taking each data pair and getting their difference, and finally RMSE is equivalent to standart deviation of all these differences. Not too complicated as it seems.
R-squared is between 0 and 1. Usually RMSE is absolute and something positive (newertheless differences between data pairs still could be negative, but standart deviation always will be positive, even calculated from negative data - it is absolute deviation). But it is possible to get RMSE value relative, knowing each data array minimum and maximum values. Relative RMSE (RRMSE) = RMSE/maxx-minn
Now, both parameters are between 0 and 1.
Best suitability (BS) would be if R-squared = 1 (ascending) and RRMSE = 0 (descending)
BS = R^2-RMSE
Weights. Sometimes it is useful to give different weights to each of these statistical parameters. It`s simple - just multiply each coefficient with its weight.
I want that RMSE is twice hevier parameter than R^2.
BS = R^2 - RMSE*2
Sometimes it is useful to add some constante
BS = R^2 - RMSE + Const
but very rare.