PEP 774: Add a footnote with calculation details (#4261)

Author: savannahostrowski

peps/pep-0774.rst

@@ -89,7 +89,7 @@ difference between the actual commits and the same commits with stencils added
 amounts to a difference of 0.03 MB per stencil file. This is a small increase in
 the context of the overall repository size, which has grown by 2.55 MB in the
 same time period. For six stencil files, this amounts to an upper bound of 0.18 MB.
-The current total size of the stencil files for all six platforms is 7.2 MB.
+The current total size of the stencil files for all six platforms is 7.2 MB. [#stencil_calc]_
 
 These stencils could become larger in the future with changes to register
 allocation, which would introduce 5-6 variants per instruction in each stencil
@@ -252,6 +252,17 @@ challenges of accessibility and simplicity, and fails to eliminate the
 dependency which was deemed undesirable at the Python Core Developer Sprint in
 the fall (the impetus for this PEP), making it a poor long-term solution.
 
+Footnotes
+=========
+
+.. [#stencil_calc] Calculated using this `Gist
+   <https://gist.github.com/savannahostrowski/c66ac62267e626d82aafa27939468e30>`__.
+   This script replays commits for roughly the past 90 days, generates the
+   stencil file for the platform for each commit, and then commits the stencil
+   file into a copy of the repository if they change. The calculation compares
+   the before and after of the repository after running ``git gc --aggressive``,
+   which is used to pack the repo (similar to what GitHub does on repo clone).
+
 Copyright
 =========