Costing

I am trying to set up a costing template for projects but am having a problem with my milestones cause an "ERROR" message in the rollup of the %Prof. Forecast (see details below).

1. How do I remove the cost associated to 0 hrs duration on a milestone so that I am not dividing by zero?

2. Is there a better field to use if I want to apply a % margin to a number?

I have set up all my costs e.g:
Labour Travel Total %Prof. Revenue Budget Actual %Prof.
Cost Cost Cost 10% Forecast Cost Forecast

Labour Cost - Cost1
Travel Cost - Cost2
Total Cost - Cost1 + Cost2
%Prof. = [Number1]&"%"
Revenue Forecast = (Total Cost/(1-%Prof.)
Budget = Revenue Forecast
Actual Cost = Input as a duration
%Prof. Forecast = (Revenue Forecast-Total Cost)/(Revenue Forecast)