Basically you have to add the volt drop for the spur. The formula is the same except that the 2 factor on the bottom disappears. Choose a value of current that corresponds to the actual load (26A for a double socket, <whatever> for a light).
You don't put two RCDs in series, it is pointless. So the existing one is fine assuming you meant to write 30mA. And assuming it gets tested.
You'll just have to juggle the figures until you are sure it's ok, if you're that close.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local