Putting money aside for a future repair is sensible. Recording that transfer as the repair expense is not. Until the bill is paid, the money is still part of your cash, even when you have given it a very specific job.
A sinking fund works best when the record distinguishes the cash balance, its intended purpose and the eventual payment.
Keep saving separate from consuming
Suppose you transfer 150 into a repair reserve for three months. The reserve reaches 450. You then pay a repair bill of 450.
The three transfers did not create 450 of consumption. They moved existing money into a location or allocation intended for the bill. The repair payment is the point at which the 450 leaves your ownership in exchange for the service.
Recording both the contributions and the repair as spending would show 900 of cost. Ignoring the contributions in the cash plan, however, could leave the main account looking more available than it really is.
Calculate the contribution from the remaining gap
Imagine a known bill of 900 due in six months, with 300 already allocated. The unfunded amount is 600. Six equal contributions would be 100 each, assuming the timing allows all six to arrive before payment.
Starting from the full bill and ignoring the existing reserve would overfund the target. Dividing by twelve despite having only six months would underfund it.
MoneyHelper's budget planner encourages accounting for costs that do not occur every month. The practical next step is to connect those costs to the actual time remaining. MoneyHelper
Do not give the same cash several jobs
A savings account may hold reserves for insurance, travel and emergencies. The balance is real, but the allocations must fit inside it.
If the account holds 2,000, you cannot simultaneously treat all 2,000 as an emergency cushion and all 2,000 as money committed to a near-term bill. Displaying several goals does not create additional assets.
Keep an allocation total and an unassigned remainder. When a bill changes, update the target and the contribution plan rather than silently borrowing from another purpose.
Review the reserve when the facts change
A sinking fund is a plan, not a guarantee that the final bill will equal the estimate. A revised quote, a delayed renewal or a changed household need may alter the amount or date.
Record the eventual payment from the account that actually funds it. If you first move reserve money back to the current account, that movement remains a transfer. The later payment remains the expense.
After the bill is settled, decide what happens to any surplus. It may remain for the next cycle, return to unassigned cash or support another goal. Make that decision visible so that the reserve does not become an unexplained pile of money.