I use .4p as my guide when assessing whether to use cash or points rather than a fixed £ cost of the room.
But sometimes I’ve gone below this because I’ve felt that I’ve already spent or will spend too much cash on a trip (flights etc) so using points is better for my wallet.
I have a spreadsheet where I calculate the value I’m getting not only per stay but also cumulatively and that allows a little flexibility in dropping below my 0.4p rate.
You also need to factor in the points you lose by not paying the cash rate and if that would affect your status. I’ve just booked a cash rate for a night that has a great points value but I need to earn qualifying points for spire next year on the basis IHG won’t give another extension.