How tight do you run your projects?

I submitted a project plan, along with costs to the senior management last week and was told, off-the-record, that my timescales were too short and the expense budget was too small.

I have been a PM for a number of years and I am often told that my budgets are either too high or too low.  At the beginning of a project, this often concerns me and I always wonder what I have missed, or what the other person knows... that they have not told me.

I have the disadvantage, in one way, of a being a contractor, but also the clear advantage of being a contractor in another. 

For the disadvantage, I do not know the project history or "norm" within the specific company.  I do not know if projects historically tend to run to time and budget.  I do not know if suppliers are particularly difficult, or is procurement can hold the ordering and payment processing up.  I do speak to the other project managers and build relationships with all parties involved, from procurement, finance, technical teams and testers etc and I do find if there are any lessons learned from previous projects, from either the project managers or the PMO department.

The advantage I have is that I know different companies work in different ways.  To mitigate this, I build the relationships between the key teams and make sure that they can accurately estimate their timescales, costs and highlight any risks and issues for me.  I of-course build in a certain percentage of time and additional cost to enable any overrun.

What are you best tips for dealing with forecasting within an unknown company?