The rule of 72 was obsolete when cheap tiny electronic calculators arrived ~ 50 year ago. For stocks, the rule is worthless anyway for several reasons. The web is loaded with free online calculators that are easier/more accurate than the so-call "rule of 72"
Who cares how long an investment takes to double? The goal should be to prudently maximize return... no matter the timeframe. Buffett doesn't have a computer on his desk in Omaha. He doesn't have a calculator either!
Note Buffett has "almost" never owned T. I'm quite certain he's heard of AT&T.
News 
Market Data 
Discover 
