Which RFM will give you a graph for VRS speeds and RoD???
40kts and 400' is far too conservative a set of figures - see Nick's graph.
The Brit Mil for many years have taught 30 kts and 500'/min as a 'beware' condition but those figures haven't changed from the days of underpowered helos with very low disc loadings and very low downwash speeds.
Modern helos (even the R22
) are much more powerful and have higher disc loadings and downwash speeds so your RoD must be much higher to get even close to VRS.