Good Morning Marek,
OK, I see your logic, but it does not match up very well with my experience.
You weigh 85 Kg. (187.4 lbs) and I weigh slightly less, but my weight has increased over the last few years from about 73 Kg. (160 lbs.) and I have not had to make any major adjustments in mast foot positioning.
Let me explain my logic on this and see if you can understand why putting the mast foot way forward on almost any board is not a good thing to do unless you are racing at the highest levels on a formula board and you want to keep the nose down and get the absolute best VMG upwind.
You've seen my suggestions on starting with the mast foot near the back of the slot and keeping it as far back as you can and still maintain control.
I would guess that you have never had the conditions where you really experienced "tailwalking".... right.
So until you see what that's about (tailwalking) you will not understand the sudden "loss of control" that requires moving the mast foot a little forward to regain control.
Here's the "phsyics":
Mast foot back allows more of the length of the board to come out of the water when planing on smooth water. If you have really flat smooth water, and good winds (perhaps you get something like this on your small round lake as there's not much "fetch" for the wind to work on making waves on the top of the water) you can allow as much of the board as possible out of the water by moving the mast foot to the back (rear) of the mast adjustment slot.
The board will plane it's fastest with this setting, right up to the point that the boardspeed exceeds the optimum speed for the fin.
The board will all of a sudden "hunt around, jump up out of the water a little, and generally act like a hooked marlin that gets up on it's tail and shakes it's head trying to shake the hook from it's mouth.
This is a control issue and the best way to "settle things down" is to move the mast foot forward slightly in the mast slot. ( 1-3 cm increments here)
What this does is move the mast foot pressure forward slightly and this changes the fore/aft Angle of Attack (AOA) of your boards planing surfaces.
Slightly less AOA results in slighty more of the bottom of the board being engaged with the water's surface, and a slight increase in the overall hull drag.
This slows you down slightly, and keeps you in control.
To go really fast, you need to be right on the edge of being out of control, so it takes a few adustments of the mast foot to find the "sweet spot" where you have "just enough"
control and can comfortably attain max. speed.
You can change the point/speed that your board "tailwalks" (if you want to go faster on the same board) by reducing your fin size and using a well designed slalom or speed type fin (usually a nearly vertical foil in a very small size).
So, putting the mast foot forward of the "sweet spot" for your particular board/rig/fin combination is simply slowing you down and making every thing more difficult as the board is basically "dragging" due to more than the necessary amount of hull in the water.
When you add chop to the mix, you may need to adjust the mast foot a little more forward, but be aware that you can put the mast foot in a spot that creates some sort of "harmonic" with the chop on the bottom of your board and get into a "bound and rebound" cycle that can be almost impossible to control. Moving the mast foot back a little more usually stops the harmonic and you can go over the chop pretty smoothly.
Having mast foot pressure too far forward in chop really slows the board down and really pounds the sailor as the board has to "slam into" each bit of chop and the "entry point" where the board first touches the chop is forward in the flatter part of your board instead of further back where there's some "V" that can soften the ride.
So, to take your sailing to the next level, I would suggest that you start out
"experimenting" on your F-Type because you are already comfortable on it and have lots of Time on the Water(TOW).
Try moving the mast foot nearly all the way back (if you have the std. twist plate or twist plate washer (11.5-12.5 cm) move it back until the edge of the twist plate/washer is almost covering the serial number. (I forget exactly where I used to put my mast foot on the F-Types, but I know it was well behind the center of the slot).
Take your 9.8 m2 rig out and see if you don't go a bit faster and the board doesn't ride a bit smoother.
Then move it back a couple of cm and see how that feels.
Then move it forward 2 or 3 cm (ahead of your first setting) and see how that feels.
When the board jumps onto a plane pretty much effortlessly, and goes fast and smooth giving you pretty effortless control, you are zeroing in on the sewwt spot.
When the wind comes up enough that you need the 7.5 m2 rig, start out with the mast foot at the same "sweet spot" you had for the 9.8 m2 and then do the same experiment.
Then do the same with your 7.5 m2 on the Carve 111.
I think you will be amazed at how much faster they go and how much easier both boards jump onto a plane.
Hope this helps,