IMO, both ways are the same ease to determine. You just stand at the side rail with your eye near the diamond and close one eye and look straight across. The center of the ball is easy to find.if no line on the table which there were never lines on tables in pool rooms back then. so that is why most places said any part of the ball behind the line makes it unplayable. this way you could lay your cue across or a piece of string and if it touches the ball its in.
how are you going to measure the center of the ball or the base of the ball on the table without an argument if its an important shot.
what is right or should be isnt important what is is what you can prove or demonstrate.
and the old, it works equally for both sides so why not.
I've never in 30 years of play seen a player put their cue or a string across the diamonds. Only the eye test.