^^^^^Just a word of precaution: If you drill holes to allow you to lube the screws take care not to get the drill chips down in the screw area.
As far as where to drill them, pick a spot about 8 inches down from the top. That will give you the chance to also lube the inner legs too. Drill it large enough to look into and run the legs down until you see the screw. Get some spray grease and give it a couple shots.
Or if you don't want to go that big on the hole, run the legs down until you can no longer feel the legs through the hole. Then spray away.
One guy says he takes the foot off the inner leg and puts some grease on a length of angle iron and holds it up against the inner screw while running the motor. Basically whatever works.
Grease zerks on the outer leg are pretty useless since the grease is not reaching the screws until you build up a large amount of it in there. But it will grease the area between the inner and outer legs.