Roger, good to hear that about your shotgun.
If you want to use a certain weight of broad head then you will cannot just try to get a certain total weight. You still need to attain a certain FOC. That is unless you are going to add weights to the front and back. If I were you I would look for somewhere around 10 to 20% FOC and be happy. A FOC too high is better than one too low. There are many discussions about the percentage of FOC but in my experience in shooting up to 40 yards its not that critical. I suppose if you want to shoot targets at 70 to 100 yards then you will have to pay more attention. There's a link for you. http://www.socalarchery.com/Information/front%20of%20center.htm
If you want total weight to be increased you are probably use a heavier arrow shaft. Again you can add weight to the front and back to acheive it but I kinda think you may want a heavier shaft because of shaft spine (stiffness of the shaft). As weight goes up the spine or stiffness should increase. You can also use smaller lighter vanes or feathers to lighten up the rear end. Some guys use epoxy to bond their inserts to the shafts to add weight. You can also use a longer arrow. Bottom line is to experiment and find a happy medium of weight, speed and range (light stuff donesn't want to fly further with great accuracy and has no momentum to pass through. Its all about a good balance of qualities that you seek.