Separate fixed and flexible costs
Start with transport, accommodation and prepaid bookings. Then set realistic amounts for food, local travel, activities, shopping and a contingency.
Use the same currency consistently
Choose a home currency for the overall budget and record local spending as you go. Treat exchange rates as estimates so small movements do not create a false sense of precision.
Review during the trip
Compare actual spending with the plan after the first few days. A small adjustment early is easier than discovering the overspend on the journey home.