Reference for calculating $R^2$ on a subset of the samples I've been looking for a method to calculate $R^2$ on a subset of the samples (a subset of the instances, not a subset of features), and found this answer from Dave. It suggests using the mean of the original samples ($\bar{y}$), rather than the mean of the subset, when calculating the TSS - .i.e.:
$R^2=\dfrac{\sum_j (y_j - \bar{y})^2 - \sum_j (y_j - \hat{y})^2}{\sum_j (y_j - \bar{y})^2}$.
Using this method has resolved the problem I'm having when using the subset mean to calculate $R^2$, where I get a low or negative value if my subset has very low variance (e.g. if the subset is instances with target values in a small range), and I'd like to use Dave's method in a paper I'm writing.
I've searched for academic references for this method of calculating $R^2$ but I have not found one so far.
Does anyone know of a suitable reference to use for this?

As my question is so far unanswered and I haven't been able to find a reference, I'm posting how I resolved this as an answer.
In my paper I justified this approach with the statement "When $R^2$ is calculated for a subset of samples, $\bar{y}$ is the mean of the measured LFMC for the full sample set, not the mean of the subset. By using this value for the mean, all $R^2$ calculations are compared to the same baseline, thus allowing comparisons between $R^2$ for different subsets of samples."
