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Long legs for an organist is not necessarily an asset. Yes, you can reach the pedals easily but what if the bench isn't high enough?
One of the solutions is to put wooden blocks on both sides of the bench. You can put as many blocks as you want and height of the bench can be adjusted even further by placing the block high end up.
Thick hymnals work even better than wooden blocks. If there are some telephone or tax books around, they will suit just as well.
But what if you don't have anything that could be used at hand and it's not your organ? You're just a guest, right? Then sit as far back as it's comfortable.
Oh, and you will need to grow some abs too!