Yep, ditto here.
Try the rig plain with what you've got. Based on what I see, you might need only 2-4 pounds. As you know, I like to thread a couple of weights right onto my rig, if I need them at all.
Optionally, there are some places that sell stainless backplates that are worth 8 pounds, 10 pounds, and 12 pounds (they're just thicker)... And, of course, they put the weight closer to your center of buoyancy, which is your lungs.
...But I think you're going to find that in a set of doubles with a manifold and bands, that your weighting is slightly head-heavy and butt-light... That is, I think you'll find that, even with perfect weighting, you're going to need to move your weighting down a bit over what you're accustomed to.
...And the easiest way I know to do that is to thread a couple of pounds on your harness.
Of course, now will be the right time to tell you that Jet fins are about a pound or two negative in the water...
So if you find yourself thinking "ankle weights," consider a set of Jets first - same idea, but without the strokiness, without the additional gear, and... You can backward kick.
Of course, to justify that, you're going to have to wear out your Slingshots - time to get in the water!