I use a 4.5" thick,slab of minicell foam to test penetration.The 1/4" plywood sounds good as does a piece of hide or leather.
The one thing to remember with flexible foams like this,temperature can change their stiffness.For this reason,if I want to test 2 or 3 different arrows,I do them in one session,so things are consistent.At least try to keep the target medium the same temperature,if testing on different days.Maybe store it indoors and shoot in the shade.
I don't count any shots that hit a previous hole.I guess a person could use a meat thermometer,to maintain a consistent medium temperature.I find 4.5" minicell foam to offer more resistance than a deer's chest.Arrows that have passed through deer,antelope and bear,penetrate about 2/3rds of the shaft length(50-54# bows and heavy arrows) through the foam.I like this as comparisons can easily be made.A pass through can't be very easily quantified and with a hard medium like wood or phone books,the differences are in fractions of an inch,which I don't completely trust.
I like to have no pass throughs (in testing)and for differences to be in inches.For me,it's ideal to use a medium that allows about 2/3 rds of the shaft to penetrate.