Hi Sean! You’re thinking about it the exact right way. The best way to do this would be to monitor your expenses now and go through them line by line to figure out which ones you likely won’t have in retirement, as well as any additional ones you might have once you retire.
The biggest expense that is hard to calculate is healthcare / long term care. This can vary a ton based on a number of factors. Depending on how far you are from retirement, it may be worthwhile to figure out the cost of the Medicare plan you may go with, as well as any additional coverage you may want. This will help give you your medical expense budget.
Remember when you calculate retirement expenses, plan on them going up roughly 2-3% per year while in retirement. This will help you factor in inflation as well to your calculations.