Rotating timetable issues

I have created a rotating timetable for groups up to 20 but now need to transfer this to actual times - each group could have a different amount of time and the number of groups can be changed. It's fine if the lessons are in order 1-20 but when these rotate eg 4-20,1-3 it's problematic for me - I have tried nesting ifs but this becomes very long and cumbersome. Would love it is someone could help with this - I have provided a copy of what I have so far