Anyone with experience with MR analysis can you please tell me how to calculate F statistics for the SNP instruments? I see different formulas, not sure which one is correct.

For example, this study describes a formula where you calculate the R2 first and then F.
https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1003288#sec008

Citation 29 in the above study actually has a slightly different formula, not sure what n here is, sample size?
https://doi.org/10.1177/0962280210394459

I found this script in the github page of MRCIEU through google where the F is calculated differently, in a simple way.
https://github.com/MRCIEU/Health-and-Wellbeing-MR/blob/master/Two-sample%20MR%20Base%20script.R

@stevesphd can u help me with this doubt pls? thanks :)