Call your local weather department or your County Extension Agent for the average date of last frost in your County. In the table you will note that mine is May 5th. Plug in your date, and then fill in the appropriate dates before and after. Post a copy of the table in every location where you work with seeds. Weeks before Last Frost is a universally understood concept; it is as important to communication as zonal hardiness. Later, you can adjust your seed starting schedule for plants that have or have not reached sufficient maturity at transplanting time by simply adjusting the sowing date in your database, i.e. adding or subtracting one or more wks_blf.
|WKS_BLF||Your Date||My Date|
My own table of dates is a bit longer than the above, ranging from week -18 up to week +25, since I sow a great variety of seed types.