I'm sorry man. The answer to this cannot possibly be this complex. Please let me explain this a different way. Imagine you have a piece of paper. And this piece of paper has 160 squares across and 25 rows of these squares going down. Now lets say you write your letters kind of fat and to write one letter on the piece of paper, it will occupy 2 squares. That means you can have 80x25 letters on this paper. Now lets say we give this paper axises. X goes across and Y goes down. Now lets say I want you to draw a letter at coordinate 10,5. But instead of counting 10 squares across and 5 down, just count each individual square. Remember that 1 letter = 2 squares. So you would count to 1610 and make your letter (cause our formula is Offset(1610)=Y * 160 + X * 5).

Now what I'm trying to do is opposite. What if I said, make a letter at offet 1610. Would you count 1610 spaces? the 1610th space is 10 boxes over and 5 down. Does this make sense?