The question of where do you put your foot is depending on what board you are riding ,
I surf a few different boards ..11.4 nalu ,even my 14 ,in smaller waves
but mostly i ride a 8,7 placid, 8'8 flow ,9.1 acid ,i surf them all different and it has come natural to me to change
stances and foot position ,the 14 and 11,4 I do lots of walking up and down the board and do stand on the outside rail to turn ,
they are big boards and getting weight in the right position is a must to turn them and gather speed ..if you where to stand at the back only you would sink the tall and go nowhere ,so walking up for gathering speed ...walk down the back for turning and stopping nose dives ,
short boards ,start from the middle and work your way back ,and most times you stay there ,unless wanting more speed for racing waves ,
The 9,1 is the faster board and i tend to ride it further forward ,as it also can stall unless being in the pocket ,
the shorter board 8,7-8,8 ..i go straight to the tail and stay there pretty much ,every board will be different ,where to find the sweet spot to put your feet ..so you go searching with your feet ..to start ..when you find it ,you remember and go back to there each time ..
we are always shifting weight from one foot to the next ..i have found most of my weight is on the front until the turning starts ,but the back foot is the steering foot ..and the bending of the knees .It is hard to explain ..it really is just practice ..i learnt SUP surfing on a 10 foot 33 wide board ...until i found it too hard to turn ..then went to a 9,6 ,then just kept going smaller .Enjoy your learning ..good time ahead .it sure is fun .
heres a picture of my early days ,it was 8,8 long X 32 ''140 litres wide board .and i noticed in a lot of photos my front foot seemed to be more towards the turning rail ..

this one on the 9,1 gathering speed ,rear foot further forward .
8,8 flow on the tail is where you need to be on this one ,