Currently, you're setup doesn't allow for any cancellation or error handling. The latter is an implementation detail, so I guess you've handled that in the actual code. I would consider the following:

• Return the started task out of Start
• Will you ever need to provide a result from the started task? If so, provide an overload for Func<double,T>
• Provide cancellation support via a cancellation token (if business logic dictates this is acceptable)
• Provide a Stop method that also that can stop the task created by Start

Currently, your setup doesn't allow for any cancellation or error handling. The latter is an implementation detail, so I guess you've handled that in the actual code. I would consider the following:

• Return the started task out of Start
• Will you ever need to provide a result from the started task? If so, provide an overload for Func<double,T>
• Provide cancellation support via a cancellation token (if business logic dictates this is acceptable)
• Provide a Stop method that also that can stop the task created by Start