How to set subwoofer delay?

[ismarketing]

On my 120, I just updated my crossover settings to deal with a nasty room mode.  I see in the configurator an option to set delay on the sub output, but I can't find any guidance on how to set it.

My Genesis 928 sub sits right behind my left Revel M20 speaker, so there is no real difference in distance from the listening seat.

Any and all suggestions are welcome.

Thanks.