Hi Mel and David,
I agree with David’s answer – m = 0.6 is within the physically plausible range of values but the reason for sticking with m = 0.8 is that many RORB models have been set up and calibrated with this value and there is a large body of literature providing regional estimates of the kc parameter that assume m = 0.8.
You may also want to refer to the excellent discussion on this issue in Section 5.4.4 of Book 8 of Australian Rainfall and Runoff, which (despite the title of the Book of ARR) discusses selection of m across the full range of flood events. Section 5.4.5 of Book 8 of Australian Rainfall and Runoff then goes on to discuss considerations for selection of m for modelling rare and very rare flood events.
Hope this helps,