I find cash and redemption rates correlate much more strongly than they did as little as two years ago. I frankly do not think there is much to be gained from trying to do exact maths.

IHG is doing points sales with an astonishing frequency. Unless you are sitting on a glut of points due to work travel, I suggest the following rule-of-thumb.

If the cash rate is > 20% higher than the replacement value of the points needed for an award, redeem. Otherwise, pay cash.

Of course, due to IHG's near-constant point sales with the 100% bonus, we know the replacement cost of a point fairly precisely.