Gear Shifting

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I have rode a motorcycle for about 6 months, 3 months of which I have spent with my GZ250. I have read the manual and at first really had to adjust to the gear ratio for shifting (my first bike was a 400). I have tried to stick to what the manual says about when to shift gears: Change to 2nd when you hit 12 mph, 3rd at 19, 4th at 25, 5th at 37. If anything, I go over this by a few mph, give or take. However, I've been reading a lot on the forums about riding in 4th clear up to 60 mph, especially when going up hills. I usually run the freeway in 5th at 65, and then when I hit a hill (which are moderately steep here in Iowa) I just crank the throttle and at worst I drop to 55 mph. Am I shifting up to 5th at the right frequency here? What is a good strategy for the high speeds/steep hills?

Thanks for the help.