If it is greater than 6 I would consider using moderately active. Just from your description though and because you have a desk job, I would put lightly active. Sitting most of the day takes away a lof of activity.
Since you stated you are going to begin weight training, I would suggest you eat back those calories on days you weight train outside of your normal activities. It is difficult to gauge how many calories one burns during weight training. I know MFP is off on my own calorie output as I have a My Fit and it tells me how many calories I burn and what is says is much more than what MFP said. MFP also said I burned in excess of 700 calories on the elliptical for only 30 minutes, which was way above what I burn as I do that after my weight training and only do it at a moderate pace and don't let my heartbeat go over 145 bpm.
But I am not expert on this as I looked into it a long time ago. Maybe Suja will have better advice. I know I label myself though as "sedentary" even though I do workout in excess of 6 hours per week, I do this mostly because I feel those 6-8 hours of exercise are minimal in regards to how much I sit at work. I also have a desk job and rarely get up more than 3-4 times per day and I usually eat lunch at my desk (talking to you ladies while I'm doing that!)\