Name the situations before choosing an amount
Consider what could reasonably change: a late arrival needing a different transfer, an extra meal during a delay, luggage storage or replacing a small essential item. A trip with several separate bookings has a different exposure from a simple stay with a direct arrival.
There is no universal percentage that makes every journey comfortable. Estimate the practical alternatives you would consider and choose a reserve that fits your circumstances. This is travel planning, not a recommendation about borrowing or financial products.
Keep ordinary costs outside the buffer
If you know you will need airport transport, breakfast or an attraction ticket, put it in the main budget. Calling expected costs unexpected makes the plan look cheaper without changing the money required.
Likewise, a planned restaurant treat or souvenir belongs in discretionary spending. Once the reserve is allocated to a purchase you already intend to make, it no longer provides room for a disruption. Keep the categories visible even if the money sits in the same account.
Do not assume reimbursement
When travel fails, ask the operator what assistance or rerouting it will provide before arranging expensive alternatives. Rights and conditions vary by journey and ticket arrangement. Official EU rail guidance, for example, distinguishes through-ticket protection from individual-ticket situations.
Keep notices, receipts and written instructions when appropriate, but do not count on every cost being repaid. A buffer should help you make a workable immediate decision while the applicable process is clarified, not depend on a guaranteed future refund.
Review after a major change
If you add another city, change to an early flight or book a remote hotel, reconsider the reserve. The likely alternatives may now cost more or involve another night's lodging. A number chosen for the original itinerary may no longer fit.
At the end of each day, update what remains without turning the trip into constant bookkeeping. If the reserve is untouched, that is a successful outcome, not a reason to spend it before the final transfer. The buffer has done its job by keeping a small problem from dictating the rest of the visit.
