I built my own spreadsheet, which I just learned I can't upload here.
This is the relevant equation:
=INT((A2-A5)*(100))/((1+A7)*100)
Spreadsheet Key:
A1; Input your Base weapon speed
a2; =A1/0.25 (Base Swing Ticks)
a3; [blank line]
a4; Input your Stamina
a5; =INT(A4/30) (Bonus Swing Ticks)
a6; [blank line]
a7; Input your SSI percentage
a8; [blank line]
a9; =INT((A2-A5)*(100))/((1+A7)*100) (Swing Speed Calculation)
a10; =(A9*0.25) <-----actual swing speed is now found here.
Let me know if this doesn't work for you.

EDIT: Here's a screenshot to make more sense of the above.