Crovevct has worked for me ok, but using the recommended dose with their gun doesn't cover much of the sheep - only the most vulnerable areas. The label says it only protects the parts where it lands, so I apply it using an empty and washed out household spray bottle such as those used for window cleaning liquid, kitchen surface or bathroom cleaner etc. The volume of one trigger squeeze can be measured by spraying water into a syringe at the start, so you can still apply the right dose, but over a much larger area of the fleece. When using the spray bottle you have move it sharpish over the sheep to maximise the area treated.
Incidentally, why is the weight related dose so critical as per the label, when it's only an external treatment?