If you have more than a 10% difference betwen your new and old lamps when you change them you need to change them sooner. A $100 light meter is cheap compared to either changing lamps too soon or not soon enough. Also remember starting in Jan. 1, 2009 mag ballast can no longer be sold in the US, all new replacement ballast will be Electromagnetic or electronic and the lamps will have to match to get the maximum output.

(Off the subject, but why does Ace have a bug crawling on his post?)