I have found that it isn't hard to simply write down the ingredients as I add them, the calories and fat grams for the quantities I use, total them up, and divide to get cals and fat grams per serving. I had to measure at first to know how many servings were in my pot I use for making soup, but now I can estimate pretty well.
I've been doing this for quite a while, and if I use low fat ingredients and mininal meat, I have consistently found that a large (2 1/2 cup) serving of vegetable, bean or potato based soup comes to about 175-250 calories and about 8-11 grams of fat. Or thereabouts. Your mileage may vary. :-)
Caroline

SW=197/sz 20 (8/22/08)
CW=157/sz 10
GW=150/sz 10=7 more pounds!!!
50 y/o, 5'9"