You need to use some sort of "wick" to rid the johnson grass. Let it grow taller than the bermuda - then run the wick through it. No bermuda will be exposed because it is too short. Neighbor uses a naked toolbar on his 3-point hitch and wraps with old sheets-towels, then pours straight round-up on it almost to saturation, then just rides over the spots killing the tall stuff.
Kind of like some gardeners who put on a plastic glove, then a cloth one over it - dip it in round-up and touch only the undesirable plants.
I would cut it now unless you think the johnson grass will be 30% or more of the total harvest. If so, just wait a couple of weeks.
Try some diquat, it will give a much faster burn than round-up. You will see it withering within a couple of hours.
I'd rather have a bottle in front of me than a frontal lobotomy.