I'm happy to help. At the moment, the ability to calculate a budgeted line item using a percentage from another isn't available in QuickBooks Online. We're constantly evolving and your feedback helps us improve greatly, so I'll submit this idea to our Product Development Team for possible implementation in future releases.
In the meantime, I recommend checking out our blog to stay up-to-date on all things QuickBooks.
Shoot me a reply if you have any follow-up questions. I'm here to ensure your success. Have a safe and productive week ahead!