Most people make a starter of 1-4 lts of wort to feed the yeast on, then pitch the resultant yeast into the 40 lts. There are a few on line calculators to work out exact numbers. Mr Malty comes to mind, the Wyeast website has a calculator as well, I prefer it over the Mr Malty site. One other option is to put 20lts of wort into the fermenter and pitch the packet then after 24 hours pour in the other 20 lts, I've not tried this option, but it's been talked about by some forum members. If your a bit of a novice to smack packs then I would use the calculators.