Lumen tester readings

Hi there,
Although I am not a newbie cultivator, I am new to this site and new to using a lumen tester. I have browsed every site imaginable to try and decode this lumen testing issue. My lights I believe are 2 years and older (600 watt Gavitas) and I say "I believe" because I sort of "adopted" this farm and I have been here close to 2 years. I have heard conflicting stories from many old school cultivators in the area. Some say they've run these same lights for 3 years and still have great production. They do, however have to keep the lights alot closer to the tops than normal. Others say a year, year and a half tops, and you should change your bulbs out. (this is also the advice given to me by the bulb supply stores in the area so im a bit skeptical)
I went and got a lumen tester, and it is reading 70.3 a foot away from bulbs and varies in each area. Can anyone help me decode this... 600watt Gavitas start out at 90 000 lumens brand new...the brand of the lumen tester is Hydrofarm digital footcandle meter.

Hello kushgoddess,
Check your power draw.
In my lab I use a kilawatt meter to measure the efficiency of bulbs. Bulbs seem to pull power in accordance to what they can handle. If your 600 is drawing less than 500 its running less than 85% efficiency.