Hi there and thanks for your question.
This is a drug for which we do not have good information as far as what dose will be toxic. However, I will give you some things to look for and together we should be able to decide what to do.
Zanaflex or tizanidine is what is called an alpha-2 agonist. It can cause sedation, and more seriously bradycardia (which is slowing of the heart rate). It would be a good idea to measure your dog's heartrate. You can do this by feeling for the heart beating on the left side of the chest just behind the elbow. Count how many beats you feel in 10 seconds and then multiply by 6. A 75 lb dog at rest should have a heart rate between 70 and 120 or so. If this is significantly less than 70 then there may be a cause for concern.
If you think your dog is sedated and has a low heart rate then I would recommend an emergency vet visit. The effects of this drug can be reversed by an injection of a medication called yohimbine.
Also, if the tablet was ingested within the last 30-60 minutes it would be good to induce vomiting by giving 1 tbsp 3% hydrogen peroxide orally every 5 minutes until the dog vomits. However, this does come with a slight risk of aspiration, so only do this if your dog just recently ingested the medication.
You may want to also call poison control for animals to be sure. They will likely have more information as far as just which dose is toxic. The number is(NNN) NNN-NNNN They will charge $60 to a credit card.
I hope this info helps! If you have more questions, just hit reply, or if my answer has helped please hit accept.