10% of the project cost. Ensure that the cost is all established to the last screw, otherwise it can get out of hand.
Interim payment should be made on Architect certificate with retention of say 2% at all stage of payments. Make sure that the payments of later stages are the larger proportion of the cost. Project managers have the habit of abandoning the projects at this stage as a blackmail.
No matter who you employ to deal with the project. Your involvement will be required at all times, irrespective of all that is in writing the interpretations are always different.
Don’t forget the project manager will also get kick backs from suppliers which will be loaded onto you estimate.
The above is a guideline and the situation can vary.