You really shouldn't be just guessing weight on foals. It is to easy to underdose and end up building resistant parasites on your property (which is already a big big issue.) Foals should be started on a deworming program at about 2 weeks of age and dewormed regularly (many vets recommend monthly) until they are a year of age. This is because of their high susceptibility to parasites, especially ascarids which can do alot of damage and who's ova can remain in the environment for 10 years or more. Rotating dewormers with foals is very important due the incidence of parasite resistance to the different dewormers---ascarids may be resistant to ivermectin and pyrantel while strongyles may be resistant to pyrantel and fenbendazole. So rotating ivermectin and fenbendazole is a common recommendation.
You should contact your veterinarian to get specific recommendations for your area of the country as he may have specific information on the resistance issues there.