mirrorselect

Default instructions in the handbook are interactive, doesn't mention it can autoselect. Here's the relevant command for, well, me.

mirrorselect -o -R "North America" --deep -s5 >> /mnt/gentoo/etc/portage/make.conf

-o ensures the tool outputs the config data to stdout.

-R "North America" makes sure I'm only looking at mirrors local to me.

--deep grabs a bit more data from each server. Necessary, since I'm on a fast connection.

-s5 selects the fastest five results.