Turbulent penetration speed is a compromise between stall margin and protection from overstress. You want to be as fast as possible to avoid stalling the airplane in low-speed excursions, but don't want to exceed the speed where you will break the airplane in high-speed excursions.
If you don't have a stall margin indication in the cockpit (as in the 747-400) or maneuvering (Vg/Vn) diagram for the gross weight range you fly in, your best bet is to use a speed in the middle of the range given in your FHB. If you have a cockpit indication, fly in the middle of the maneuvering margin, as long as that speed is within the FHB limitations.
There will always be some uncertainty. For example, the 747-400 turbulence penetration speed is 290-310 KIAS or .82-.85 IMN. Cruising at FL310+, the KIAS range and IMN ranges do not coincide. However, .84-.85 IMN will keep you in the middle of the maneuvering margin displayed on the PFD.