Keep an assumption register next to the model to make the business case survive finance

The fastest way I have found to make a business case survive finance: keep an assumption register next to the model. Every input gets a customer-side owner, a source, and a confidence rating. When finance pushes back, you negotiate one assumption at a time instead of defending one big number. It also kills the end-of-quarter surprise where the champion admits the headline figure was “something sales came up with”. The model stops being a sales artefact and becomes the customer’s own document, which is the whole point.