Cash flow reflects budget
Monthly cash flow and end balance is calculated taking budgets into account for accurate financial planning.
If the budget applies to more than one account perhaps give the option of which one should have the budget reflected on its cash flow.
Lastly, as you add expenses for the category, the Cash-flow or end balance isn't altered until you overspend, as the remaining budget automatically updates itself.
Perhaps if you add a transaction belonging to the budget but on an account which isn't the one in which Cash-flow is reflected, the remaining budget is updated and the end balance on the account in which the budget is reflected increases (as that money is no longer reserved for the budget)