I hope this message finds you well, present circumstances excluded. I am a licensed attorney with over a dozen years of employment law experience. It is a pleasure to assist you today.
The short answer is that you are likely fine taking the approach that you are taking relative to the approximation for workers comp as long as you can show, if audited or questioned, that the nine hours that you give these individuals is approximately on average what they are working.
In order to do that you need to be able to show, in some capacity, that their outside sales requirements on average, results in a 9 hour work day. This should be re-producible via some form of documentation. If you cannot produce some form of documentation that approximates the 9 hours, then you may need to go to another system...such as a call in system wherein they call in to a call center to note they are on the clock and then they call in when they go off the clock. If they are on sales calls in a company vehicle, you can purchase CAD systems that are GPS monitoring devices that can evidence whether they are actually moving or on location relative to their stated calls.
So, there are methodologies all the way around here, but in short, as long as you can show that their average outside work day approximates 9 hours, then you are good to go.
Let me know if you have any other questions or comments.
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Best wishes going forward!