Rule 5. If your current action results in less than doubling, then you may use as much of the original stake and profit to complete this step. In short, on any one step you don't have to double the money in one transaction. You can carry out as many transactions as you like, until you have succeeded in doubling the stake money.
Example: You are on step 18 (£640-£1280) and you manage to do a deal which increases the £640 stake to (say) £900. You may now use that £900 in another deal to try and reach £1280.
Rule 5. Detail. For example, you are on step 14. You invest £40 and only get back £60 instead of £80. That's okay, you can use all £60 (or less) in an attempt to make the extra £20. If you make more, then rule four applies. You don't have to complete each step in one transaction or 'deal', although the earlier steps will almost certainly be 'one deal per step'. Later steps will almost certainly be several transactions per doubling.