You can set the directory for the cache in the "root" of Preferences > Folders.
Personally I don't compute the tile cache as a ratio of RAM but as a ratio of "free RAM under normal circumstances": OS loaded, background apps running, browser open on usual sites, etc... And then I use a ration of "1" (ie, take all RAM noone else is using).
You can have a few clues about Gimp's use of resources with Windows > Dockable dialog > Dashboard.
Personally I don't compute the tile cache as a ratio of RAM but as a ratio of "free RAM under normal circumstances": OS loaded, background apps running, browser open on usual sites, etc... And then I use a ration of "1" (ie, take all RAM noone else is using).
You can have a few clues about Gimp's use of resources with Windows > Dockable dialog > Dashboard.