The Guidotti-Greenspan-IMF rule entails:

adequacy of foreign exchange reserves in terms of the 'imports cover of three to four months' of a country's imports

countries to bold 'liquid reserves' equal to their sho1t-term foreign liabilities (maturing within a year)

conttries to hold 'liquid reserves' equal to their short-term foreign liabilities (maturing within 6 months)

adequacy of foreign exchange reserves in terms of both short term external debt and a measure of the scope for capital flight (pan of M2) modified by a 'probability factor' captured by a country risk index