Ah ha, okay I apologize as it was setting a Random Seed that I couldn't get working with the Make Validation Formula. The option to set a Random Seed disappears from the UI when selecting a formula. But, I just found that Random Reset can solve this:
Names default to here(1);
dt = Open("$Sample_data/iris.jmp");
//Make a grouping Column
dt << New Column("Group", Numeric, "Nominal", Format("Best", 12), Formula(Floor((Row() - 1) / 5)));
random reset(10);
vals = transform each({r}, 1::nrow(dt), row() = r;
Make Validation Formula(
[0.75, 0.25, 0],
<<Stratification Columns( :Sepal length ),
<<Grouping Columns( :Group )
)
);
New Column("Validation with Grouping", Numeric, "Nominal", Format("Best", 12), Set Values(vals), Value Labels({0 = "Training", 1 = "Validation", 2 = "Test"}), Use Value Labels(1));