Another thing to consider is the "JD Blue" needle definition itself. There are many different JD Blue needles. I'm going through this now with my brother's Husky 250 with a PWK 38 swap. The needle I use is from a kit that used to be called the "PWK38-39 MX/Enduro generic kit". Now it is specified for a KTM 144SX. I told him to contact JD and have him clarify it is the same needle. From my experience JD has always suggested that you start rich and work leaner which is a good conservative plan. Sometimes the starting points are a lot farther off than others. That said, the jump over stock main size is primarily due to the fact that the JD needle, like the D and C series single tapers, has a much wider tip and obstructs the nozzle more at full throttle. Standard practice is one or two sizes up over stock. Countering that, 300s have a bigger signature, so compared to a 250 in the same conditions running the same needle I would expect the results to be one to two main sizes leaner.
FWIW here in the north east USA we ride very tight and technical terrain, and for the most part use very little of a 250s power. Our bikes have to be jetted clean off the bottom as well.
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