Hi Steve, there were a few things I concidered when I had the 16 gallon tank made that I put under the back seat in Red'sGold, knowing it added a hundred pounds back there, and because I moved the motor back, adding more weight to the stern. I don't know what will work for anyone else, especially depending on how they want to use their boat, but I can tell ya what worked for me on that boat, and I ran it for one season, took it to a couple shows as it was designed, but sporting a little more horse power.
Moving the motor back added weight to the stern ... I removed about 200 pounds weight off the motor and rebuilding the back seat reduced another 30 pounds
Raising the drive reduced trim effect ... moving 100 pounds of gas from the bow to the back seat gained bach trim effect
I never ran that boat with the back tank before I done the mod to the transom, and installed the bigger motor, so I can't give spec's on that, but I do know that adding more weight to the stern, with-out reducing weight will set the boat deeper in the water, more time to get the boat on plane, but probably no more than a 10 year old in the back seat.
I usually just kept 3 or 4 gallons of gas in the bow tank and ran out of the back tank. With 240 pounds of gas in the bow tank made 3 mph difference in the boats speed. Choppy water, 240 pounds if gas in the bow, in my opinion, caused a lot of stress on the hull, keeping more boat in the water, rougher ride. Little gas in the bow tank, running off the back tank allowed the boat to skip across the waves more on the stern, much better ride.
When I took that boat to Red Wing I had the 16 gallon back tank full and 25 gallons gas in the front tank, that seemed to be a good balance for that boat, even with 5 people on board, decent planing time, good ride and good speed.