First off, cheers for getting onto a bike, getting in shape and so quickly seaking out informtion to improve on how you're training. To that end, hope this helps:
The fact that your maximum heart rate has decreased with training is a normal physiologic response to aerobic exercise. No need to go out and try to reach 200+bpm any more. Whether you should train by 180bpm or not is hard to say, it depends on how you reached that number. I can say that that is a larger decrease than I would expect.
Performing an incremental test to exhaustion (after several days of easy, recovery riding) is the best way to determine a maximal HR that is applicable to the formulation of training zones/intenisities. IOW, just hammering for a couple of minutes until you're about to puke and looking to see what your pulse is doesn't work out the same way (not saying you did this, just for contrast). Check out our page on this topic:
http://www.cyclecoach.com/articles/?article=Power_Guidelines&ext=.htm