Here is a tutorial that I did a while ago on making your own weights:http://www.duckhuntingchat.com/forum/viewtopic.php?f=15&t=201083&p=1794813#p1794813
I think it is important to keep a couple of things in mind. First, a spot that has 2 feet of water at low tide, might have 12 feet of water at high tide, so make sure that you not only have enough weight, but that you also have enough line. Also, this time of year, ice can be a major issue. No matter how much weight you have, if there is any ice flowing, it will probably ruin your day.
As far as weight goes, I use 4-6 ounces on my ducks and 8 on my geese, but I'm not hunting any significant current. I would probably double that amount if I were to hunt bigger tidal rivers. The amount of weight that you will need to hold bottom will depend on the style of anchor. You can probably get away with less weight if you use an anchor similar in style to the guys fishing the outer banks. They are a pyramid style sinker, with prongs, so that they can grab the bottom.
I don't hunt big spreads in the marsh.... probably 6-10 decoys. I find that black ducks tend to stick in smaller groups, and since many of the mallards have moved on, blacks are the primary target. So, more weight doesn't really affect me, since I'm not lugging around a couple dozen decoys. I think experimenting is key. Find what works for the areas that you are hunting.