Would it be okay to provide a template when asking reference? I am applying for grad school this year and I am considering sending an email to ask a professor to write me a reference. He had previously agreed to write me a letter because I took one of his advanced courses. 

I am not sure if it'd be okay for me to offer to provide a template to him for the letter. The reason I want to do this is I will be applying for a PhD in a different, though quite related, field, and I want to make sure that he knows what the advisory committee will be specifically looking for in his letter, and make sure that his letter corresponds well with my statement of purpose.

Is this an acceptable practice between professors and students? Or would they consider it offending since he is supposed to provide his independent opinion? And if it is okay, how can I offer it in an appropriate way?
Personally, I would be somewhat offended if a student gave me a template for a letter, and I wouldn't use it.  (Unless I had asked for one, which personally I would not.)

However, it is perfectly appropriate to say, "I believe that the committee will probably be interested in my ability to dangle participles and reticulate splines; it would help if your letter could address this."  It is also fine to include a list or resume of your other activities that he may not be aware of, but leave it up to him how to use this list.

It's quite possible that your professor has written letters for students in this different field before and already knows what is expected.  Also, for letters which are submitted online, the writer normally gets a message from the requesting institution explaining what issues should be addressed.