I will skip all the usual stuff about "weighty" issues and cut to the chase. Boy, is that out or character.
I have tried science and empiricism on this issue and the latter seemed to have worked better (I got great grades in differential equations, but arithmetic was tricky at times.) . In part, the answer depends on what the dive circumstances are, but the most complicated is probably diving from an ocean-going boat; so, I will describe the technique I use there and you can modify it to your "swamp cabin" or whatever.
If you use, 6 or 7 lbs w/o a wet suit, try 12 lbs with a 3mm suit. However, put 4 of that 12 in the form of a 2lb and 2-1lb wts in a BC pocket. Now, hop in the water and try a buoyancy test: i.e., let the air out of your BC and try to descend. If you go down like a rock, put air back in your BC and put 2 lbs back on the boat. Now, try again. If you still descend like a rock, put one of the 1 lb wts back on the boat and repeat. And you should be done, but you can always dump that last 1 lb...however, you will be at 8 lbs, just 1-2lbs over your non-suit wt and I do not think that will come to pass.
By the way, when I am using a new suit, I go through the same process, but if I would be holding people up by going back to the boat, I just leave the 2-lb wt at the anchor or mooring point and pick it up on the ascent.
And you will discover that as your suit compresses with age and you get more experienced, you will need less wt. So do a buoyancy check every so often as you do more diving.
Hope this helps....
Joewr
