See Jerkstrand+2015 (Late-time spectral line formation in Type IIb SNe), section 5.12.
Fe II 1.26 mu can be used to estimate the Fe II mass (Eq. 9) if a reasonable temperature can be determined. One way is from the [Fe II] 7155 / [Fe II] 1.26 mu ratio (Eq. 8).
Note that there is an error in one of the last sentences of section 5.12, the intial 56Ni masses would be ~0.04 and ~0.1 Msun, not 0.19 and 0.05 Msun.
The method gave for these stripped-envelope SNe Fe masses correct to a factor ~2 (as the 56Ni masses are well known and should be close to the Fe mass at 200d).