Certified 100% Organic
Many people have an objection to lecithin because of the solvents used (and removed) during it's processing. If that is the case, or you just wish to keep organic, this is your option. A bit more expensive since it's a bit tricky to make without solvents, but that's to be expected.
The addition of soy lecithin acts as an emulsifier and viscosity adjuster. You will probably see it on most chocolate labels. It is purely an optional ingredient. The industry uses it for consistency batch to batch. If you do want to use it, approximately 0.2% of any additional cocoa butter you add is about right.
For an interesting effect you can add more as it will also affect the mouth feel of your finished chocolate. A little bit will go a long way.
And it is especially helpful if you are going to be baking or cooking with your chocolate. Addition of about 1% will virtually eliminate the chocolate 'breaking' into an oil slick when combined with liquid ingredients.
If you just want to shoot from the hip, 1/2 t per pound is a fine amount to easily add.
1 oz is about 3 T of lecithin