Is there a way to decrease fee by increments in automation?

Hi, I'm trying to create a task to reduce fee for channels that haven't been routing for 24 hours. 
In the channel policy I only see a set ppm rate.
When trying to set the rate as "-15", this caused some kind of overflow and instead set the fees at 4294967281 ppm. Is there a way to reduce fee by increments which I'm missing on?
Thanks