Pappy,
I would think that there are several ways to doing your cook on the Weber kettle but unless you have done this kind of cook before, I would suggest setting the cooker up with 1/2 of it direct and the other half indirect. Assuming that you will be grilling about 40 burgers and probably needing them all done at a certain time, I doubt if a fellow could pull it off with a cooker setup for all direct cooking. Something to consider is that it will take about 3-4 minutes to give the burger some grilling marks on the direct side and then you will need to move them to the indirect side for a few minutes to finish cooking. Starting with just 10 burgers, by the time you can load them, the first ones will be needed to be turned over. By the time you get them turned, it will be time to start pulling them off the cooker or trying to find out where to put them. Even with just these 10 burgers, you will be busier that a one armed paper hanger! You will also need googles as the smoke will be everywhere with the drippings going down into the coals.
If it was me, I would load a Weber charcoal chimney full with lump charcoal and after it was ashed over, pour it into the kettle just using 1/2 of the grill for my direct cooking. Leave the other 1/2 open for indirect cooking.
With 40 burgers, I would plan to sear about 8 at one time as this is about all a fellow can take care of and still get a good product off of the grill. After searing, place them on the indirect side and start 8 more burgers. Keep at this but do watch the burgers on the indirect side and pull them off at an internal of 165 degrees. And I do think the one Weber charcoal chimney of lump will be enough for your 40 burgers.
I think that when I got them all done, I would just flat guzzle that first beer.