an inexensive software package called CIVIL TOOLS does that calulation plus about 25 other routine CE calculations for you. Its about $300 find it on the web
One item that has not been mentioned is the fact the water from your french drain needs to leave the site. If you can't drain the pipe to daylight ie doownstream and away from your house, you will need to install a sump pump then pump it to your roof gutter drain. You do have roof gutters don't...
