Mechanics of Retirement Planning

A question I posed to some of my classmates who wanted to practice DCF or time value of money questions:

John is 20 years old and is making $100k annually. He plans on retiring at 65 and will require 70% of his current annual income every year thereafter. The general life expectancy is 'til 85. And he can earn 8% on any money he invests.

How much does John need to save per year in order to retire at 65 according to plan?

[Solution]
You would break this question into two parts.

Part I: Nest Egg - Understanding the PV of the money he needs in retirement (aka how much he needs to have saved by 65)
Using a financial calculator:
PV = ?

FV = 0 (Doesn't plan on giving any inheritance when he dies)
PMT = 70,000 (70% of 100k)
P/Y = 1 (1 period per year, NOT semi-annual)
I/Y = 8
N = 20 (Retires and lives for 20 years, 85 - 65)
PV = 687,270.32 (How much his nest egg should be at 65)

Part II: How much he needs to invest every year
PMT = ?

FV = PV @ 65 = 687,270.32 (How much his nest egg should be at 65)
P/Y = 1
I/Y = 8
N = 45 (Works for 45 years, 65 - 20)
PV = 0 (Starts with nothing)
PMT = 1,778.16

John needs to save 1,778.16 per year. Now most people will say: "That seems really low"

That's true, but look at the scenario: John is saving for 45 years and spending for 20 years. For all of those 65 years, John is earning 8% on every dollar he's invested (aka he's not using). This returns to our original point when it comes to personal finance: "You can buy stocks. You can buy bonds. You can even buy good advice. But the one thing you can't buy is time."