He may well be right for a specific film/dev combination as they all differ. +50% time is a big expansion for most films though.
Best way is to expose a couple test strips from a wedge and run a densitometer over it. That will give you a definitive answer on the contrast you're achieving.
If you're struggling with ZS concepts, get "Beyond the Zone System" (BTZS) by Phil Davis. There are plenty of secondhand copies floating around cheap and you can read it cover-to-cover in a couple nights assuming you already have a passable grasp of mathematics.