Yeah that would work. It all depends how mono you want the bass to be. I think you can isolate the frequencies below the threshold using one of the monitor buttons on the right. So have a listen and check that there is nothing below the threshold that feels like it could sit well in stereo. Sometimes you can set it lower, like 100-150Hz. It all depends on the synth and the song. The slope just sets the slope of the transition between mono and stereo at the threshold you have set, with 12dB being the steepest.

The width control sets the width of the frequencies below the threshold. If you just want to narrow the low frequencies rather than mono them, you can set the width a bit higher.