Extent sizing question.

Here's a better picture of what I am trying to calculate on my tables I've run an oncheck against a table to see what data pages are begin used and then divide that by the number of months since creation to get a monthly growth rate. Then I multiply that by 12 to get a year. This will give me a years growth. How can I fine tune the 1st extent so I am not wasting space? Tks Ed