Wider bars & shorter stem.

(I believe) A general rule is to take off half of what you've added to the bars from the stem, e.g 700mm bar and 100mm stem to 800mm bar, 50mm stem. I heard it on the internet so it must be right, and my bike certainly feels fine after doing so.

Might be interested in taking the Nukeproof bars off you then if they are the 31.8mm ones

There is no forumula that works really - is all personal preference. Try what you have and experiment.

Or you could just move your cheeks forward on the saddle a bit, does the same thing and is cheaper.

Moving the saddle forward doesn't achieve the same as fitting a shorter stem.
The stem effects your position when out of the saddle as well, which is probably more important than your seated position because it's the position you are in for more technical riding and has more effect on handling.
With wider bars you will need a shorter stem (unless it was too short before) but the right length can only be found by trial and error. Try cheap ones from ebay or borrow spares from mates before buying a better quality new one.

Wider bars do not need a shorter stem. The stem need to be a certain length to give you the "reach" you need. The stem length is affected by top tube length and seat tube/headtube angles. Saddle should be set in one postion normally for men the back of the saddle is 120mm from the centreline of the seat post.

On my own 29er though I like a very forward positon (lots of reach) and I use a 120mm stem with a 750mm wide bars. Gets a bit tight sometimes on some race courses.

supersonic is right about the personal preference. Try the wider bars and then try different stem lengths until you find one you like. You will eventually learn what you like and then your bike will have the stem you want on it first time.