Two dates and one figure
Two dates and a figure will tell us most of what we need to know about today's Budget.
The first key date will be when the chancellor expects the economy to return to positive growth. In November he was hoping for the summer. Now it is likely to be early next year - but the timing will be important. The latest consensus forecast is for modest growth in 2010 of 0.3% - but that consensus is falling fast. If the chancellor comes in below that, or even with negative growth for next year, he will be able to say he is being cautious.
A slower return to growth will spell bad news for the borrowing forecasts. We learned this morning that net borrowing for the 2008-9 financial year was £90bn - a little less than experts such as the Institute for Fiscal Studies had been predicting but still £12bn higher than forecast in the pre-Budget report last November. The figure for this year and 2010 will be higher still - at least £160bn.
Growth deferred means debt piled up. As we know, that purely "cyclical" borrowing is not the chancellor's biggest problem, because most of it should go away by itself as the economy recovers (though it does raise the long-term cost of servicing the public debt).
But if the economy is going to return to trend later than forecast, that will at least mean that he can push forward the date at which he expects to get the debt ratio under control.
That is the second date to watch for. In November Alistair Darling said he would stabilise debt and balance the current budget (excluding investment) when all of the global shocks of the past two years had worked their way through the economy. Back then he thought that would be 2015-16.
Look to see how far he pushes that date forward today - and remember that the further away it gets, the further it will be going into not the next Parliament, but the Parliament after that.
And finally, that figure I'll be looking for? Of course there will be plenty to choose from - for example, the level at which debt is expected to peak as a share of GDP, which the chancellor reckoned would be just over 57% in November. I would be surprised if he doesn't raise that by at least 10 percentage points today, probably more.
But the number I'm going to single out is the forecast real growth in total spending from 2011-2016. In the PBR he was looking at average real terms growth of 1.1% a year during that period. Will that turn into a real freeze in spending, which would undoubtedly mean real terms cuts in spending for services that people care about? We'll have to wait and see.
It is possible he will keep that figure unchanged - after all, why make things any easier for the Conservatives than you need to? But he will need a fairly convincing story to tell the markets if he does, inevitably involving tax rises. All will be revealed at 12.30 today.