How an independent Scotland might design and organise its armed forces is the topic of robust debate in the light of the looming referendum.

Most objective commentators accept Scotland could raise and run its own armed services. The discussion focuses mainly on whether it should.

Most participants in the debate have taken the easy route to come up with models for Scotland's defence. It is convenient to say, variously, that they would be roughly 10% of the UK inventory; like the armed services of roughly equivalent countries like Denmark or Norway; or based on the average European percentage of GDP and apply it to Scotland's share of the UK defence budget to arrive at a result.

All of these approach the question from the wrong premise. My co-author Richard Marsh and I have attempted in our report The Blue Bonnets: Defending an Independent Scotland (RUSI, October 2012) to approach the topic in the manner taught in the military.

We first looked at the threats and risks. What would Scotland have that others might covet or attack? Then we looked at what, in terms of military forces, Scotland might need to defend against those risks, air, land and sea. Finally we costed the model we had arrived at to see if it would be financially feasible.

We made a few assumptions. We hypothesised that the focus of an independent Scotland's defence policy might be regional rather than global, as would befit a small nation of 5.2 million people. We also assumed Scotland's defence policy would primarily concentrate on defending itself and its interests but would have at least the option of contributing to coalition military options if it so chose.

As for what Scotland would wish to defend, there are no blinding flashes. Aside from maintaining territorial integrity and protecting its citizens at home and abroad, oil and gas resources are foremost, plus fishing grounds, sea trading routes, physical and electronic communications, and such industrial assets as remain such as shipbuilding. Nobody in their right mind thinks Scotland would be at risk from conventional military attack, and the threats might be much more likely from cyberwarfare, terrorism and organised crime.

Against that background we designed a model comprising what we thought Scotland might need, not what its government might want. Our model suggests an air force of 60 aircraft of all types; a navy of 20-25 ships and lesser vessels; and an army based on two brigades, one deployable and one primarily for reinforcement and home duties. This suggests a personnel strength of between 13,250 and 16,750 for all three services, depending on detailed organisation and equipment, and probably comprising 75% regulars and 25% reservists.

How would such force levels be raised and equipped? Essentially from Scotland's share of the UK inventory. Detailed negotiation would be required to get the right balance, with cash in lieu of assets where appropriate (for instance Trident, for which there is no need or appetite in an independent Scotland). Many Scots serving in the UK armed forces might choose to stay where they are, but others would be persuaded to join, especially if better pay and conditions of service were on offer.

We estimate our model would cost, at the upper end, roughly £1.84 billion a year capital and running costs, approximately 1.3% of Scotland's GDP. This compares with the Scottish defence expenditure of £3.3bn in 2010-11 and the SNP's recently declared budget of £2.5bn a year.

There are three questions to ask: would armed forces in an independent Scotland be necessary; would they be feasible; and would they be affordable? We believe we can answer yes with confidence to all three.

Stuart Crawford is a defence commentator and former regular army officer. He is a speaker at the second Enlightening the Constitutional Debate event run by The Royal Society of Edinburgh and the British Academy, on the theme of defence and international relations at the RSE on May 29. By ticket only, booking via the RSE website.