Thanks for reaching out to the Community for support. I'm happy to provide some insight into adding to your budget by a percent.
You can change the line item by a percent in QuickBooks Online, however, you'd have to calculate the total manually. For example, if you add 5% to the line item, QBO will change the percentage to .05 but won't increase the total. The ability to add the percentage and it calculate for you isn't available. I can see how this would be beneficial for you and your business. If you'd like, I can submit a feedback request to our product development team, so this could be considered in a future update.
You can follow all of our latest happenings and updates using our Blog Site.
Additionally, I'm including an article covering using budgets in QuickBooks Online that you may find beneficial below.
Please let me know if you have further questions or concerns by hitting the Reply button below. You can reach out to the Community or me at any time. We're always around to lend a helping hand. Have a great day!