Rye bread: proof until soft
Having your bread split isn't necessarily a sign that your dough is underproofed. Many breads get considerable oven-spring, that is expansion due to the expansion of air and last gasp of yeast from the heat of the oven. If your crust hardens before the oven spring is complete the loaf may split to release the pressure. There's a couple of ways to prevent this:


Slash your bread more deeply before baking. The goal of slashes is not just cosmetic, it allows the bread to expand
Introduce moisture into the oven to keep the crust from hardening too quickly. A pan of water works well for this, you put enough water to last for the first 20 minutes of baking. After 20 minutes the oven spring should be done and you'll want the crust to harden. You can also bake the bread in a closed pot in the oven for the first 20 minutes to trap moisture, then take the lid off for the rest of the baking