Urgent, how can I auto increase a number and lettler from cell to cell

Hi,

I am creating kind of a dashboard which it is mainly used to keep track of activities. So I am using a form which will add a record into a cell under different categories which I already have defined in a sheet. however I will like also like to automatically add a number to each which will classify that task. The number needs to always be a decimal so it can not be a whole number like 1 or 2 but it has to be 1.1 or 2.2. Any Idea how to do it? Now another tricky part is that I also have sub categories on the records and categories are not numbers but lettlers, such as "a","b"... and I will like to increment them also each time I add a new record.