The stock 18/21/28 forks are 710mm from the centre of the wheel spindle to the top flat surface of the forks. The RVF 400 forks are the same length. What will change the geometry is the amount of static sag you have on the new forks (probably less) but it won't be a huge difference in feeling.

You also need to take into account the rolling radius of your front tyre and yoke offset when calculating the geometry as the rake varies, as does the trail, depending on the combination of the above.
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NSR300R - Why did i ever have a 250...