rsquared[list___, model_] :=
 1 - SquaredEuclideanDistance[list[[;; , 2]],
   model /@ list[[;; , 1]]]/
  SquaredEuclideanDistance[list[[;; , 2]], Mean@list[[;; , 2]]]

data = {{0, 1}, {1, 2}, {2, 4}, {3, 9}};
mass = LinearModelFit[data, x, x];
rsquared[data, mass]
(*0.889474*)

mass["RSquared"]
(*0.889474*)

model[r_] := 2.6*r
rsquared[data, model]
(*0.888421*)