2^64 daltons (30 micrograms) is already about 2 gigajoules.
4!! photons -- specifically the 21cm photons corresponding to the hydrogen line [which, speaking of precision, we have the frequency of that down to around 1 part in ten trillion] -- is about 0.6 Joules.
So 4!! hydrogen atoms gets you about 1 gram, 4!! H-I photons gets you about 0.6 Joules, and the wavelength of the H-I photon is around 21 cm.
It all seems a bit marginal. Nobody actually uses the definitions to measure anything anyway, least of all by hand or memory, so it's not going to make anything more convenient having fewer digits in the still-arbitrary conversion factors.
4!! photons -- specifically the 21cm photons corresponding to the hydrogen line [which, speaking of precision, we have the frequency of that down to around 1 part in ten trillion] -- is about 0.6 Joules.
So 4!! hydrogen atoms gets you about 1 gram, 4!! H-I photons gets you about 0.6 Joules, and the wavelength of the H-I photon is around 21 cm.