#1 the wattage of your bulbs is determined by the size of the ballast running them. A larger ballast will run a lower wattage bulb higher but it will not last as long. The rating at which the bulb manufacturer publishes is that at which the buyer will receive the most bang for your buck.
#2 What Charlie suggested is accurate yet you may have the wrong approach. You need to return to the LFS
that you purchased your rock from and ask for a sample of their water for you to inturn take home and test with your kit not have them test with their kit. you need to establish a control which will then in inturn help you determine if your test kit is accurate or not. ie if the LFS
water tests at 500 and they indicate that their test kit showed 400 then you know that you are most likely 100 off. Also remember that with a young tank you will encounter diatoms and some other occurences with a new tank that you would not see with a used tank.