Do you have the stock forks anywhere?
You can measure the lengths from the axle to the top and compare, then subtract the RS forks from the stock ones, and follow the settings recommended from the NSR-World tuning pages.
If your triples are modified or not stock, then you need to see if they are lower than the stock ... if they are a different offset from the axis of the steering stem then you're going to have to do some more involved geometry calculations.
I've got the old RS250 front end (non-USD) and it is frighteningly shorter. I am going to set it 5mm recessed into the top triple clamp (I.e. -5mm showing) and I'm still going to be 8-9mm shorter than the HRC settings still.
I'm still waiting on somebody pointing me to some frame geometry measurements so that I can make a rake and trail calculator.
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MC28 SE -sold-
MC21 with RS250 engine -for sale-
MC21 race bike -soon for sale-