= 151.7(2.7)\rm MeV$ and 162.8(2.6) MeV at $M_\pi = 296 \rm MeV$ and 411 MeV, respectively. The physical decay constant, $f^{phys}_\pi = 132.7(5.5)\rm MeV$, is obtained from analysis of data with pion masses up to 411 MeV. Physical value of the squared charge radius, $\left<r^2\right>_{phys}$, is calculated at the physical pion mass for the SU(2) case and and at the physical pion and kaon masses for the SU(3) case.[]{data-label="table:PFF_ChPT_NLO-fpi"}

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We now investigate the choice of the decay constant to be used in the NLO ChPT fit. With an uncertainty of order $O(p^6)$, the decay constant in (\[PFF\_SU2\_NLO\]) and (\[PFF\_SU3\_NLO\]) can be chosen to be $f_\pi$ measured at each pion mass. Table \[table:PFF\_ChPT\_NLO-fpi\] shows results for NLO fits using $M_\pi^2/f_\pi^2$ as the expansion parameter. We observe a large difference in the results depending on whether one uses $f$ or $f_\pi$ for the SU(2) case and $f_0$ or $f_\pi$ for the SU(3) case. The fit results for $l^r_6$ and $L^r_9$ if one uses $f_\pi$ are larger than those of the fit using $f$ and $f_0$ by 40 to 60%, which raises the values of $\left< r^2 \right>_{phys}$ at the physical point by 20 to 30%. Predictions for the charge radius from 