The rule of thumb I use is $150 per hour if travel is not more than 30 minutes one way. We live in a rural area and 30 minutes covers a lot of territory; but some customers can be 2 hours away...

One fair way to calculate this Tom is to figure what you would normally be "making" in the clinic during that time out.  Please note there is likely a difference between what you "bill" for and what you "collect".  We have found it safe to say that our time away from the clinic is worth about $125-$150/hr.  This may or not include prep time for your service (once written, you may use it again somewhere else!), travel time, etc.  Hope this helps!