CFD sizing with volatile indices - how do you scale in?

Been looking at the DAX $DE40 lately, specifically through CFDs. The swings can be pretty wild, especially around economic data. I'm comfortable with the general principles of risk per trade, but when you're anticipating a move and want to scale in or add to a position, how do you adjust your initial size to allow for that without blowing past your risk threshold too quickly? Do you treat each 'add' as a separate trade with its own risk, or is it all part of a larger planned allocation?