Ah, if it were only that simple.....
I rode F2 for quite a number of years (before Starboards came on the WS scene)
and I never could get the "move it forward for larger sails....back for smaller sails" to work.
There are too many variables that you need to balance for it to be a simple matter of moving the mast foot forward for larger sails and back for smaller sails.
Most boards have a "sweet spot" and to get the best speed and smoothest ride you need to keep the mast foot near that sweet spot.
But the "sweet spot" may move depending on the following important factors:
Sail size (and type of sail)
Sailing discipline( wave; slalom; freeride; freestyle; formula racing).
Many older race boards (racing longboards) had an on the fly/on the water adjustable
mast foot. If you raced, you quickly found out that putting the mast foot all the way forward to increase the waterline length of the board was very necessary to get good upwind speed.
Conversely, if you were on a reach or running downwind, moving the mast foot nearly all the way back was nearly always the "fast way".
So, now we have shorter wider boards, and no longer have an on the fly adjustable mast foot, so it becomes necessary to first find the "sweet spot" for a sailor of your size, with your rigs.
Usually it's best to start at (or near) the back of the mast slot.
If the board is hard to control, move the mast foot forward in about 1-2 cm (3/8"-3/4") increments until the board settles down.
Now you are "zeroing in" on the "sweet spot" (for you, with your weight/stance/rigs in your kind of conditions).
Once you find the sweet spot for a particular rig size on your board, try a larger rig.
Start at the previous "sweet spot" and work from there.
Often, you can move a larger rig forward a little and get slightly better performance, but at some point, you will get to a point where the board and the sea conditions get a bit out of balance and the board begins to "pound" into the chop.
Then you need to move the mast foot back a bit.
You will "search" for the perfect mast foot postions for your various sails and boards forever.
Mark where the sweet spots are, so the next session you don't have to start all over.
Make small adjustments as the condtions (windspeed, chop, tide or current developed chop, etc.) change.
You are looking for the best position for the mast foot that gives you "just enough" control that you scare yourself a little, but you can handle it.
If you settle the board down too much (by moving the mast foot forward of the sweet spot by too much) you will lose some speed and probably begin to encounter a hard (pounding into the chop, or a pound then rebound over the chop) ride.
If you keep the mast foot too far back, then you won't have the control you need to really "send it", and you will be backing off all the time to maintain control.
So, you have to experiment, and find out where you are "comfortably fast".
And, having someone else set up your kit may or may not be a good idea.
If they are more skilled, they will be able to make your board "too fast" for you to handle and you won't like it.
If they are significantly different in weight or height, this will affect where and how their sailor weight gets to the board, and again you won't like the way they have it tuned.
It's not because they didn't tune it correctly (they tuned it for themselves, obviously) it's just that you are a diffferent height/weight or you have a different stance and the whole package doesn't balance out the same.
So, enjoy experimenting. It's the only way.
Oh, and a common myth (since formula boards came on the scene) is that you can run the mast foot right at the front of the track.
This is true, if you are a formula racer, on true formula gear.
But formula racers need to get to the upwind mark first, and they will put up with alot of
"shenanigans" on other points of sail to get to that upwind mark first.
Also remember, when you are tuning, that mast foot forward often makes you board more "sticky" and will slow you down where as mast foot back generally frees up your board and helps it to be faster, So, you need to learn what "sticky" feels like, what fast feels like, and what lack of control because the board is too free feels like.
Then start "fine tuning".
Hope this helps,