How to track revenue from variations?

How to track revenue from variations?

As a company working in Architecture and construction, a considerable fraction of our revenue is derived from additions to existing project contracts.  Currently we have no way of tracking these variations in Zoho, so they throw off our pipeline and revenue forecasts.
It is not uncommon for variations to increase a project value by 100% or more (such as with the addition to our contract of following design phases, for example)
We need variations to a contract to appear in the project total, but also show in the Revenue for the date of the variation so our targets can be tracked.
How does Zoho achieve this?

Thanks