  The question of the determination of the proton charge radius $R_p$ from
electron scattering data led to an unprecedented experimental effort for
measurements of the electric form factor of the proton at low and very low
momentum transfer in electron and muon elastic scattering. On the basis of
basic properties of densities and fitting bias considerations, a procedure is
developed in order to evaluate the impact of forthcoming data on $R_p$.
Particularly, it is shown that a 0.1\% precision on these future cross section
data is necessary to establish indisputably the $R_p$-value as determined from
lepton scattering. The ProRad (Proton Radius) experiment at the PRAE (Platform
for Research and Applications with Electrons) facility in Orsay is further
discussed, especially the experimental method to meet this stringent
constraint.
