You definitely should weigh them. Also, maybe try talking to your vet or the shelter. You can also Google it and find a few different recommendations.
This is what I have been using for my fosters: http://www.2ndchance.info/orphankitten.htm
How Much To Feed:
Each day the average kitten needs 30-32b milliliters of formula for every 4 ounces of body weight. During week one give about 15 ml for every 100 grams (3.5 ounces) body weight. During week two give 15-18 ml for every 100 grams body weight. During weeks three and four give 20 ml for every 100 grams body weight.
Of course, search a lot of different sites and talk to people that know, they all say things differently and talk to vets, and no matter what the "books" say, base it on your kittens!
Just out of curiousity, have you tried to catch mommy? My fosters' mommy is feral, too (though the rescue LIED and told me she wasn't...
) and she is here taking care of them. It's a challenge for me to deal with her, but better for the babies.
Thanks for saving their lives!! Enjoy these times- it's a been mostly wonderful for me.