Through a little bit of experimentation... pulling the power back to 1700 rpm on my C150 results in a nice 3 miles per 1000 feet descent profile; whilst maintaining cruise trim setting. That way I do not need to think about it.
And each 100rpm equates to around 50 to 70 feet per minute... so I can throttle up and down for adjustments as I need to.
In the configuration I am also travelling at 1.5 Nautical Miles per minute; so reducing to 1400 rpm will give me a minimum of 150fpm extra rate, which gives me at least an extra 300 feet on the profile per 1000 feet, or 30% extra altitude loss; returning the power to 1700 removes the extra descent rate, and then returning to cruise power removes it all together.
it is VERY rule of thumb but works and I don't need to think about it in the descent, just do it!