Kunin et al. (2018) took on the heroic challenge of modelling the species-accumulation curve based on the 200m2 X plots covering Britain and then attempted to upscale to the whole of Britain and predict the size of the British flora. The amazing thing is that 3 techniques came very close!
Kunin, W.E., Harte, J., He, F., Hui, C., Jobe, R.T., Ostling, A., Polce, C., Sizling, A., Smith, A.B., Smith, K. and Smart, S.M., 2017. Up-scaling biodiversity: estimating the Species-Area Relationship from small samples. Ecological Monographs.