If you don't use the Own Production module, you track own production directly on the production code. Don't add a contract on the code; let the budget from the estimate drive the final forecast, and correct along the way with expected additions based on what has been incurred.
How to do it
Don't add a contract on production codes with own production. The budget then drives the final forecast.
Keep an eye on the posted cost on the code, which comes automatically from the accounts.
If you see that the code will end higher than the budget, add an expected addition on the code via Add → Expected Addition.
Make sure costs are posted continuously to the correct production code in the accounts.
Tips for large projects
Split own production into several production codes by phase, for example foundation, building shell and interior fit-out. When a phase is finished, you know everything has been expensed on that code. You then have the actual result for the phase and can assess the rest more precisely, instead of one large lump item that is only settled when the project is finished.
When should I use the module instead?
If own production is a large part of the project and you want to track quantities and hours month by month against the estimate, the Own Production module gives better control. You then accrue the work and log actual consumption every month.