[authorityofnil]

I don't think this argument makes any sense. Just because there can be better uses for additional clean energy, does not make the additional energy any less clean.

By this definition, no source is clean, because there are always "better uses" for energy.

[adrianN]

The honest way is using the average CO2/kWh for the grid you're plugged into (including the CO2 emitted for constructing the power plants with some discounting function) to estimate how much carbon your synfuel releases.