(I hope this file attachment thing works)
The attached spreadsheet will calculate the distance from the stringer to the rail as you move from the tip/tail of the board toward the middle. What you do is enter:
1. The length you want between the end of the board and the point where you want the curve to become parallel to the stringer.
2. The board-widith stringer-to-rail distance that you want (in feet).
3. The angle that you want the curve to start out at (a smaller angle gives a narrower tail, larger angle = wider tail).
What this does is give you rail offsets from the stringer at 3" intervals. You can plot these on the blank, connect the dots, and then adjust based upon your gut instinct and where you want to be.
There are many uses; for example, if you're doing a squashtail or swallowtail, just start plotting at the calculated offset that corresponds to your tail width. For me, this is a tool to get in the ballpark.
Have fun and let me know what you think.
For the mathies: What this calculation does is plot a compound curve between two defined x-y points. The degree of arc of the curve will change at a constant rate between the two points. The specified starting angle will define how fast the curve needs to change in order to reach the end point.
https://swaylocks7stage.s3.us-east-2.amazonaws.com/s3fs-public/spiral_calculator_xls_4b76ec6c9c.xls