Welcome Zebra. I agree with Pat, try and make it a third but your bit sizes don't always allow for perfection so somewhere between 30 and 40 % of thickness. Same goes for tenons in mortise and tenon joints. Instead of making a tongue and groove joint I usually make a spline joint instead. A spline joint is basically a double groove/floating tongue joint. In other words, it's basically the same joint except that the tongue is a separate piece. Spline joints are easier to make and you don't lose any wood in making them. As Pat pointed out you lose on the tongue side of the joint in T & G.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.