First of all, the following numbers will not be exact. My mind is pretty much mush right now and I'm not sure I could work out the math correctly for your 6.75" mold (even though it's a fairly easy process).
Just to give you an idea of what the increased pressing weights should be (this is from a chart that I have):
15 lbs. = 40.5 lbs.
30 lbs. = 67.5 lbs.
50 lbs. = 112.5 lbs.
15 lbs. = 49.5 lbs.
30 lbs. = 105.5 lbs.
50 lbs. = 176 lbs.
15 lbs. = 56.5 lbs.
30 lbs. = 120 lbs.
50 lbs. = 202 lbs.
As you can see, as the wheel size goes up, so does the pressing weight.
I know this sounds like a lot but the higher weights are fairly easy to accomplish, considering the type of press that you have.
If using the "four post, dowel/board" type of press you can buy concrete paving blocks that are approx. 14" square x 2" thick, which weight about 22 lbs. each. Since these are large, flat and square they are fairly stable.
This method does work okay, but the stacking and unstacking is a major pain and (at least on my old press) I was constantly having to make minute adjustments to the blocks to keep from ending up with a lopsided cheese.
The easiest and most efficient way (plug ahead) is to check into one of Carter's pneumatic presses which makes the process as painless as possible and completely carefree.
Now I simply set my air pressure...flip a switch and walk away. It really is that easy and you always know exactly what pressure is being applied.
I know I write a lot about this press design but it really does deserve the high marks I've given it. It's not like I'm working on a commission program. I just believe in this design and each time I use it I am more convinced.
As I said, the numbers are not exact for your size of mold but at least they will give you an idea of what we have been talking about.
Hopefully Carter or Wayne will chime in soon with the exact numbers.
Hope this helps....