View attachment 91654
Once you annualize the depreciation, the inverse is the total life. 2% = 50 years.
Since effective age varies, this number is only good the day of the value - or year if you will- just like cost varies, sales prices vary - etc. It's not fixed. Therefore, a house that is 80 years old and updated can have an effective age far below its actual age.
If you know the contribution of the improvements by subtracting out the site improvements and land, then you can determine the RCN and see what % of the building remains. You will always have to make an assumption, just as you do in the sales approach. So, it's a choice. Estimate by your observed effective age? Or, estimate by your total life estimate (from the cost book for instance.)