Assume, your monthly target is $1,000. Now, you are taking 30% upfront money. So, if you able to get $3,500 projects a month then you can able to get 30% upfront money that equals $1,050. So, you can fulfill your target.
Here the key is buffer projects and you need to set milestones with buffer timeline. So, once you get the first 30% then target the next milestone or 30% to get within 20 days time. Then, even if client delays you still have buffer time.