Low refirgent usually means that you have a leak. It would take a set of
gages to tell if you have just a marginal leak, or a major one.
Step one is to see that the at rest pressure is between 72-114 psi for a
fully charged system. I'd use a "sniffer" to see if I could find a leaking
fitting or hose if the pressure is low. If the pressure is below 24 PSI. you
should really pull a vacuum on the system before a recharge. ( -29.9hg)  A
full recharge after pump down normally takes 21.2 oz or 600g of refrigerant.
During the recharge, stop between 14 and 20PSI, and check for leaks. When
1/2 or so of the refrigerant by weight is used, the engine is started, and
the rest of the refrigerant is valved into the low side.

THIS IS NOT A STEP BY STEP SET OF INSTRUCTIONS.
Only key information peculiar to the Miata was included. The servie manual
describes the process in full, and should be used if at all possible.
Otherwise, a general automotive HIVAC  manual should  give most of the
needed information, cautions, and how tos.