Driver Sliders and Live What-If: Change One Number Without Breaking the Model

Most what-if exercises in finance happen in a spreadsheet, and most spreadsheets have a quiet failure mode: change one assumption and something three tabs away goes stale, because nothing forces the whole model to recompute in the right order. You find out the balance sheet doesn’t balance anymore only when someone else notices, usually in a meeting.

Rexfin’s modeling engine is built so that can’t happen. Every projected number comes from a set of typed drivers (growth rates, margins, working capital assumptions, and the like), and moving a driver triggers a full recompute of everything downstream of it, in dependency order, deterministically. The engine computes every number. The slider just tells it what to change.

What “deterministic” buys you in practice

Two things, mainly. First, speed: the recompute is fast enough to feel like a live conversation rather than a batch job, moving a driver and seeing every dependent line update happens in a fraction of a second, not after a spinner. Second, and more important, correctness: the same accounting identities that hold in the base case (assets equal liabilities plus equity, cash ties across the cash flow statement) are enforced on every recomputation, not just checked afterward. If you flex a driver and the model would otherwise go out of balance, the engine won’t quietly let that slide.

That second point is harder than it sounds, because real filings don’t perfectly cooperate. The line items you can extract from a filing don’t always sum exactly to the printed subtotal: there’s a real-world gap, often from items like right-of-use assets, deferred tax, or reserves that a filer bundles into a subtotal without breaking out separately. Rexfin doesn’t paper over that gap by quietly plugging cash or forcing a balance. The remainder is carried forward as an explicit bridge line, labeled as what it is and held flat under projection, so the model stays honest about what’s actually driven by your assumptions and what’s a residual from the source filing. Cash itself stays derived the way it should be: from the cash flow statement, not from a bookkeeping trick to make the balance sheet close.

Scenarios are deltas, not rebuilds

A named scenario in Rexfin isn’t a separate copy of the model. It’s a sparse set of driver changes layered on top of the base case: remove this revenue stream, hold this margin flat, push out this hire. That’s what makes it possible to stack assumptions and compare them cleanly: a downside case and a base case are running through the identical calculation logic, so any difference between them is a difference in assumptions, not an artifact of two people building two spreadsheets slightly differently. It’s also the foundation for sensitivity analysis and goal-seek (asking what a driver would need to be to hit a target) because both are just automated ways of running the same deterministic engine repeatedly.

Catching drift between preview and save

A what-if you preview live and a scenario you actually save are two different moments in time, and the underlying data can move between them: a new filing gets processed, an actual gets restated. Rexfin re-runs the same calculation at save time and compares it against what was previewed. If anything material has shifted (the base figures, the computed years, the driver values themselves), the save is refused with a specific reason, and nothing gets persisted silently on stale numbers. You’re told to reopen the what-if and look at the current figures rather than unknowingly locking in a scenario that no longer matches reality.

Where this fits and who it’s for

This is the same deterministic engine behind natural-language what-if questions: the AI translates a plain-English question into driver changes, but the arithmetic underneath is identical to moving the slider yourself. It also sits on top of the three-statement projection engine and can start from any saved forecast baseline. It’s built for FP&A teams and CFOs who need to answer “what if” in real time (in a board meeting, in a lender conversation) without trading away the rigor of a properly tied-out model. See the rest of the Rexfin product tour for how the pieces connect.