Underdosing horse wormers can lead to resistance, this is because if a horse is given a lower than indicated dose not all of the worms are killed, so the worms that survive may and become more tolerant to the drug in the wormer.
Weight tapes are a useful tool, they measure the girth of the horse in estimated kilograms, but if you do not have access to a tape it is possible to use a formula to estimate your horses weight by measuring the girth and length of your horse and then using a formula.
Girth- measure the horse behind the elbow and round the withers in centimetres
Length-Measure from the shoulder diagonally to the buttock in centimetres.
Weight in Kilograms= girth(cm)2 x length (cm) / 11877