One of my more ambitious headlines...if only it were so easy.
It isn't, of course. A perennial problem with planning is that so often there are far too many reasons to game the process. Revenue is the biggest classic problem here - some people will want to minimise the plan to make it easy to outperform (and earn big bonuses); others will want to maximise it to make the higher ups happy.
You can't fix that yourself. But you can take steps to help the business as a whole navigate the process.
And the best way to keep everyone as honest as possible is to rely on data. Specifically, on key drivers of the business.
In our revenue example, for many businesses the obvious predictors of revenue are sales headcount and marketing spend. You will likely have at least one, and often many more, years of figures for how those correlate to sales.
It can be very powerful in reporting on a plan to simply include comparatives for how the ratios for this plan compare to prior years. Then, there is a followup conversation - "You're forecasting X, but last year you delivered Y with Z headcount." Followed by different questioning depending on whether the Y/Z ratio is higher or lower.
There's all sorts of wiggle room for sales to talk about seniority and ramping and turnover etc of course, but it's a start.
Similar with marketing spend. "So, you're forecasting a 25% uplift in revenues - but no change in marketing spend?" Again, there will be reasons/excuses, but at the very least you're putting numbers onto the uplift that the business is hoping for from a new product/channel/campaign/whatever.
Politically, all that can be difficult, so while I'm all for more junior people putting themselves out there, this can be a little bit like pointing out that the Emperor's new clothes are fake; you might want to consult with senior finance management before you action this one!
Or, to start with, limit yourself to the numbers on the reports, and discuss any interesting findings within finance before going further.