I wish I could find all the range data I found when testing the SdKfz 251's 28cm rockets. To learn them, I too highly suggest you go offline and test the range and impact spread of the rockets. I know if you sit on one end of the runway and fire the lessor degree arc to the other end you will impact at the end of the runway (small airfield, iirc).
HTC recently upgraded the damage to them {loud applause!} and they are worth taking if 2 things happen: First, you NEED supporting cover. Meaning, if you do not have a wirblewind or a tank to come with you to protect you while you find level ground and a straight line of sight you will be eaten alive by a wee defender. Secondly, you learn how to gauge and aim the rockets and know what they can do. If you can find the flag and then find the "town square" or at least the "flag corner" you can unleash some major butt woopin'.