Hey Patrick, hey Rob, hey Woodpecker, hey all,
F: Set Feed Rate
Fx - set the feed rate to x. x is usually in machine units (inches or millimeters) per minute.
The application of the feed rate is as described in the Feed Rate Section, unless inverse time feed rate mode or feed per revolution mode are in effect, in which case the feed rate is as described in the G93 G94 G95 section.
G20, G21 Units
G20 - to use inches for length units.
G21 - to use millimeters for length units.It is a good idea to include units in the preamble of each G-code file.
Often feed rate is given in m/min, e.g. here in this feed rate table, this is 1000 * mm/min.