Sounds like you really are very close - as long as you have depreciated asset values in the balance sheet then this is basically correct. The 'add back' refers to the tax computation which is a piece of work that sits between the accounts and CT return. You are then allowed to deduct capital allowances from your profits before arising at profit for tax. As you have incurred some capital expenditure i would recommend that you engage an accountant to help you with the computation and tax return this year, which by itself should not be too expensive.
Let me know if i can be of any further assistance