Mr. Ay concluded the practical aspects of the problem. In my opinion it's all you need.

I would add some theoretical aspects you may detail yourself.

There are modes with frequencies higher than the frequency at which the spectral acceleration returns to the Zero Period Acceleration (ZPA).

As frequency limit, 33Hz is included in REGULATORY GUIDE 1.60 "For frequencies higher than 33 cps. the maximum ground acceleration line represents the Design Response Spectra."

NUREG/CR-6645 says "The contribution of modes to the total response is most accurately and efficiently treated by static analysis of the "missing mass" (i.e. system mass not participating in the modes with frequencies below f_ZAP) multiplied by the ZPA".
However, it's not a simple math multiplication; APPENDIX I of NUREG/CR-6645 presents a mathematical descriptions of the "missing mass" contribution to total response. Studying also the references presented in CR-6645 would give you more knowledge about the subject.