While the soap is heating, put your fragrance ramekin onto the scale and zero out the weight. Measure your fragrance or essential oil into the ramekin. A good starting place is .4 ounce of fragrance per pound of soap. (Generally I use about 2-3% per weight of the soap.) You can go up or down from there if you're using a light or strong fragrance oil.
For this recipe, I'm using just plain lavender essential oil at about .4 ounce per pound. (A total of .8 ounce for the two pound batch.)
Once you've measured your fragrance and removed the soap from the microwave, slowly add the fragrance to the melted soap base and gently stir.