Number of points necessary for 20% uncertainties on mass characterisation, for 1 M_Earth (upper panel) and 2 M_Earth (lower panel) and different OGS contributions: GRAhigh (orange), SGmed (red), SGlow (brown), ALL_{GRAhigh,SGmed} (green), and ALL_{GRAhigh,SGlow} (blue). The dashed lines correspond to the configurations including GRAlow (same colour code as in Fig. 1). Stars indicate that even with our largest number of points the uncertainties are in fact higher than 20% (lower limit for N_obs). Diamonds indicate that even with 180 points the uncertainties are in fact lower than 20% (upper limit for N_obs).