As posted on Chia Networks blog, Feb. 22, 2021 the best ram settings for plotting chia are:
With Bitfield: Min 2500 Med 3400 Max 6750
Without Bitfield: Min 1400 Med 3500 Max 3990
According to Bram Cohen, in Modern Finance podcast, below the minimum settings, plotting will fail. And above the maximum is just a waste of ram (eg. the plotter will not use the ram, so no use allocating it).