Stem Lengths

Hola peeps,
Have updraged my bard from a 680mm bar to 760mm bar, and i think i need to drop stem length a bit, but trying to figure out what length. currently running 70mm and thinking with over 80mm of increase on the bars i should drop the 20mm on the stem and get 50mm but then a middle option 60mm does exist and its not like i've gone to a 8000mm bar :S what are peoples thoughts on this?
Cheers guys
Have updraged my bard from a 680mm bar to 760mm bar, and i think i need to drop stem length a bit, but trying to figure out what length. currently running 70mm and thinking with over 80mm of increase on the bars i should drop the 20mm on the stem and get 50mm but then a middle option 60mm does exist and its not like i've gone to a 8000mm bar :S what are peoples thoughts on this?
Cheers guys
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Dirt Jumper Dmr Sidekick2
http://www.ebay.co.uk/itm/Outland-Freeride-Dirt-Jump-DH-Downhill-MTB-Mountain-Bike-Cycle-Handlebar-Stem-/280892262446?pt=UK_sportsleisure_cycling_bikeparts_SR&var=&hash=item41667bc82e
Was thinking of trying a couple but was thinking i would have to spend £40 on test stems
Parktools :?:SheldonBrown