Large projects are likely to require some sort of require dedicated accounting system, although conceivably Gantt Charts and financial management accounts can easily be administered within a spreadsheet system given sufficient expertise. If you don’t know how to put together a basic financial plan, get some help from someone who does, and make sure you bring a good friendly, flexible financial person into your team – it’s a key function of project management, and if you can’t manage the financial processes your self you need to be able to rely completely on whoever does it for you. The spreadsheet must enable you to plan, administer and report the detailed finances of your project. Create a cost line for main expenditure activity, and break this down into individual elements. Create a system for allocating incoming invoices to the correct activities (your bought-ledger people won’t know unless you tell them), and showing when the costs hit the project account.