See a recent Boeing presentation on the problem - Boeing Slippery Runways "Update on landing performance", downloadable
here.
The problem is not as simple as it might first appear.
Selected text from the accompanying notes:-
With manual brakes, the brake system applies whatever brake pressure is commanded by the crew. If autobrakes are used, the autobrake system targets a predefined deceleration level and will apply the wheel brakes only as required to reach target level.
In summary, the deceleration from reverse thrust is always additive when using manual brakes, whether on a dry or a slippery runway.
Conversely, the deceleration from reverse thrust may be additive when using autobrakes, depending on the autobrake setting and the runway conditions.
The Advisory landing distance data contained in the Boeing QRH includes the effect of reverse thrust for Max Manual braking and for Autobrakes (whether additive or not).
As a reminder, reverse thrust becomes the most effective deceleration device as runway conditions deteriorate.