It's a tricky one isn't it........ math has never been my strongest point but what works for me is batting some points at the more expensive Hilton's....... I often stay in London Bankside and the Waldorf these are nearly always £220 plus....... if I bat say 30k of points at the price it brings it down somewhat.
I'm not sure if this is the optimum bargain , probably not but i kinda like doing this.....oh and it's still cancelable.