You actually do have a gauge on your hand for jelqing. The gauge is built into your penis. Here's how:
When you squeeze, your glans expands. Look at the expansion on your glans very closely. When you have weak glans tissues, you'll see white patches, and at times, different color like darker shades of pink or bluish to dark purple, rather than nice uniform color throughout your glans. This are hotspots. Hotspots are known for causing sudden blistering. The reason why the color changes on the skin is the soft tissues, or scarring tissues, the glans is attempting to rejuvinate and reinforced. The patches need more oxygen in the blood than usual, and the blood tends to be closer to these newer spots, staying far away from the lower tissues like the rest of the glans. The closer the blood is to the natural air, the fast the oxygen being consumed or oxidized. Because of this, color changes occur.
So, how hard to squeeze? Based on the gauge of your glans. If you have a nice uniform color all over your glans, you can increase
SLOWLY upwards to a more moderate level. If you have a nice dark pink color all over your glans, you are now reaching 6inHg. Why? You're pushing larger blood volume that pressurized against the upper layers of the fascias (your upper tissues layers).
If you can still feel comfortable without any pain, and no issue with hotspots that can cause blister, increase the intensity slightly more aggressive while fully erected. You are now entering between 8inHg to 9inHg, close to the upper bound of safe internal pressure. You'll be surprised how faster your blood color is changing while at this pressure.