Create segment based on random % of another segment

Particularly useful for warming and re-warming IPs, it would be very useful to send an email or create a segment based on a random % of another segment. What I am doing currently takes hours - picking a random variable like 'contains the letter z in email' and waiting 5mins for a refresh to see if it got me to the # of users I need for the warming. This feature would also be particularly helpful for testing - create a campaign that you want to test on a % of your segment.