Accounting Project Manager's Time in the Project Plan

HI

I have the following scenario. I have a PM whose annual salary let say is $120K and his salary is coming out from the project budget that he is managing. In the project plan the PM is also assigned to some task in the planning phase and also is involve in monitoring and controlling of the project. The project is a 12 month project.

My question is how best to manage the PM cost in my plan? If I put as a fixed cost and pro rate and that will shows $10,000 per month. This is fine BUT what happens during the period when he was assigned some tasks in the planning phase which is based on the hourly rate from the resource sheet. I will be adding his cost twice during this task period if I have the fixed cost pro rated as well. 

Basically, I want to know how best to account for PM cost in the plan where he is also assigned some task in the plan. Hope I make any sense here. Any inputs on how  best to handle this much appreciated. Or if anyone know of any good article written on this subject of PM's time accounting on a project please let me know.

best rgds