It is alwys best to consult with your physician prior to use if you are pregnant or nursing, or if you are taking any other medication. It is always recommended that you consult your doctor before starting any diet or nutrition modification as part of a weight loss program as they are familiar with your medial history and any prescriptions you may be on.
Thank you for your interest in Lemongrass Essential Oil for laundry soap. I'm sure it will smell wonderful. Essential oils are very concentrated. Their safe use requires they be treated with care and respect. The user should be knowledgeable about their properties and actions before any use. Most personal applications require drops rather than ounces.
Unfortunately I cannot provide you with the exact amount of drops to use, however experimenting can give you a mixture that will work well for you and your family.
Here is a recipe provided by the manufacturer for a lemongrass skin tonic that might give you an idea for the ratio of essential you'll nee for your detergent.