Enter data, drop down 1 cell, enter more... etc...

Guys quick one I hope... the google search on this keeps showing me drop down box info, so I guess my terminology isn't good...

I've a variable set from another function, called "PTYCNT" it's value is 3 (in this instance)

I want to ADD data (from another calculation) into cell M4, and go down by "PTYCNT" cells adding data as I go, but only by the number of cells returned by the variable (which will never start anywhere other than M4, and never go below M16).

Hope someone can help, I could post code for this ( an example sheet maybe ) but I'm not sure it would be clear without more info than the above.