A few basics answers got from Mel in response to your questions.
1 rotation in square foot garden is instant.
one plant comes out,fresh compost added new plant or seed is planted.
this can the next plant in a rotation or succession.
2. Grids are important. to keep square foot organized.
do not use string tried to nails for you grids. string will will brake when you are not looking, you will scrape arm on rusty nail next season. wood or plastic slats should for grids will last.
3. To avoid potato volunteers dig all soil out square in question then sift soil.
4. 50 cm compared 30 cm for broccoli square foot method has 2 solutions.
plants overhanging square is fine. You just under plant the a plant next square over
or let it over hand the side of box.
5. The spacing between boxes should be at least 2.5-3 ft to allow easy travel to take into
account plants the over hang the box.
more info later.
