Apart from calculating for the weight sitting atop the tripod, make allowance if you want to mount the camera when the centre column is used horizontally. I use that when ever I want to shoot any thing on a table or in a flower bed. If your body + lense + ball head weights 3kg (in my case it is 2kg), then the tripod should be rated for at least 5kg in normal use and at least 10kg if the centre column is to be extended horizontally.
