Health service chief executives have been fond of likening the balancing of the NHS books to landing a jumbo jet on a postage stamp.

It is a rare and welcome break from their general preference for football and cricketing metaphors for which the general gist is “what’s £500m either way between friends when the budget is £100bn?”.

Fair enough (unless of course you are a part of a patient group which might have benefited from that £500m, or a hospital seeking a new building, or a politician at a smaller ministry where £500m goes a long way thank you very much. Or a journalist with a newspaper to fill).

For 2013-14 however, it looks like a new metaphor may be needed - something along the lines of trying to land twice as many jumbo jets on a moving stamp - with unlicensed pilots.

Much consternation has already been expressed about the handing of the NHS purse strings to countless inexperienced commissioners from next April. But, as this was a political decision, not much can be done about it.

However, NHS finance directors are finding it harder to accept the moving stamp end of the metaphor - otherwise known as the gaping £1.5bn variation in the estimated size of the national specialist commissioning budget, which currently veers between anything from £10.5bn to £12bn. This matters to nascent clinical commissioning groups because the bigger the specialist commissioning budget, the smaller their own, and a potential £1.5bn margin of error is quite a lot when the postage stamp CCG commissioning budget is only around £60bn.

Landing on the final specialist figure is proving tricky because the answer will depend on the answer spat out by the complicated set of algorithms currently being run in an attempt to determine which (and how much) hospital activity will fall under the widened definition of “specialist” care and hence be commissioned regionally rather than by CCGs.

A bigger specialist budget will of course imply a narrower CCG purview, but that does not mean commissioners can feel comfortable charging ahead regardless: commissioning plans need to be finalised, posts filled, contract and service agreements signed. Only after that can they even start to think about lowering their landing gear.

Sally Gainsbury is a news reporter for the Financial Times, sally.gainsbury@ft.com