hi,
the reflection being acceptable or not is really a matter of design. Usually for narrowband systems that are not size limited (such as your design). the S11 should be well below -10dB (~-15 : -20). Since you cannot guarantee getting the same results in the measurement.
However, -10dB corresponds to 90% of the incident power is radiated, -8dB => 85% radiated, -3dB => 50% radiated. Thus, for me, as long as I can ensure that the high reflection won't affect the previous stages (e.g. drive the power amplifier out of its stability region) and I am not designing a low power system, I would accept -8dB and maybe even much worse. I will consider it a poor design but it can be used.
best regards,
Adel