You now have the ability to control the maximum percentage of your Segment budget going towards specific campaigns within that segment
In order to do this, first select the goal that you want to modify, and go to the Campaigns tab.
From here, select the 3 dots to the right of the campaign you want to limit the budget on, and select Limit Budget Allocation. You then have the option to limit the budget of this campaign from 1-100% of the total segment budget.
Note: This is a limit on budget allocation, meaning that if you set 50%, that campaign can use up to 50% of the total budget, but may use less than this.