For the actual trip it was a simple but quite time-consuming matter of researching the costs of transport, visas, accommodation and food in the countries we planned to visit, and trying to estimate how much of our rental income would be left after we had paid agency fees, insurance, maintenance etc. The aim was a cost-neutral trip, ie we would try to travel within our monthly income, averaged out over the 18 months. The conclusion was that it would be best to spend a long time in India and forego Australia entirely – and that was what we did during the first 18 months. Then we thought “stuff it”, extended the trip, and raided the savings to go to Japan and Australia. But at least we knew what it would cost so it was an informed decision.
For the long term I created a spreadsheet with our savings as a starting point, then fed in year by year assumptions about earnings (I actually did this about 5 years before we gave up work), rental income, mortgage, expenditure, tax and pensions (we obtained updated forecasts), trying to be pessimistic about both incomings and outgoings. It was really helpful in deciding at what point we could afford to give up work without our savings becoming uncomfortably depleted. It’s actually the kind of thing that was worth doing even if we didn’t travel, just to check our retirement finances were OK.