Due to the angles involved you want to be removing no more than half of the distance the bike was lowered, so you're looking at 10-12.5mm roughly.

Best thing to do is (with a mate helping you support the bike), take the bike off the side stand and use a piece of wood propped under the side stand pivot. Cut the wood down in stages until you get the desired angle of lean. When you find what you want measure the length of wood from it's base to the centre of the side stand mounting bolt. This will give you an approximation of the overall side stand length required.

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