I agree with the Easton way.It gives you foc in regard to spine and that is the most important reason for measuring foc,along with arrow flight.
Steel or solid aluminum will not spine the same as carbon or aluminum arrows so why would you add the tip in when trying to get an ARROW foc right in relation to spine and flight.
I do agree that with a mathematical equation that adding the total point length would be correct but this isn't a mathematical equation for mathematical sakes.It is for spine and arrow flight sakes.
Besides,Easton kinda makes up the rules for this don't they.They should know how their arrow performs best and why they reccomend certain foc for different kinds of shooting.