I want to make a platform that has 2x4s for legs.

I did this once before using a single extra-long screw (4 inches, I think?) to attach the legs from the top of the platform, drilling straight down into the center of a 2x3 (I drilled a pilot hole first). It's held up pretty well.

But it was pain to get everything lined up (I couldn't chose the positioning of the legs until I was on site, and it was hard to mark the position of the tiny pilot holes on the underside of the platform), so I thought that using my Kreg jig might work. That I could drill the pocket holes in the 2x4s, and then simply draw the outline of the 2x4s on the underside, and then take everything off to the side and attach.

(the last one, I made w/ 2x3s; I thought I could use 2x4s for this one, since I'm drilling through them; they might be stronger than 2x3s.)

How do I position the pocket holes in the 2x4? And, what setting on the jig do I use? I was thinking it would be smart to have two screws, maybe even one from each side, going in opposite directions.

Got any advice?

(this is quick-and-dirty construction; I want it to be sturdy, but it's not supposed to be pretty)

(this platform is going over my DD's big cast-iron radiator in her dorm room, so the legs will have to fit into position around the pipes on the floor. I'm hoping the 2x4s won't be too big to fit in there.)

(I still can't believe the first one is holding up so well--it holds a bookcase. And it shifts a little, but not by much, and the wood around the screw is still completely firm. This one isn't supposed to hold anything nearly that heavy.)