Due to the problems with increased resistance to wormers don't worm ad hoc. You should only worm when its required. Sheep will start to scour (get diarrohea) or look ill, their behaviour will be out of character for them; and in the case of the barber pole worm, haemonchus, they will become anaemic; when they need worming.
You should get a faecel egg count (FEC) analysised by your vet lab and they will be able to say what type of worms you have and the most effective wormer for your particular worms. Most vets will supply you with the correct amount of the wormer in the case of needing small amounts which would apply with your number of sheep. The amount of wormer you give goes on the weight of the sheep or the heaviest sheep if you need to do all.
The white wormers are the only ones that treat tapeworm, you also get yollow and clear, plus the combined ones (orange, purple) which with your number or sheep you should not require.
Depending on the season and weather conditions depends on what worms are around at that time, so take a poo sample from each animal who requires it - i follow mine around with a supply latax gloves or small plastic bags, scoope up and turn glove/bag inside out and knot; then let the lab sort it out!
But take advise from your vet and if poss don't worm adhoc
Hope this is some help