Pacing for 90km time trial with heart rate monitor
For those unfamiliar with the CTS system, there is a guide here

The answer is a hard one without knowing the individual athlete, as a better trained athlete will be able to tolerate much more work in the tempo and sweet spot ranges.

On race day HR is often a few beats higher and you can squeeze out that bit more due to a combination of adrenaline and motivation.  I would be looking to target
86-90% for the first half and push on to 88-92% in the 2nd half if you feel you have more in the tank.

Putting the HR monitor to one side, it is also useful to pay close attention to your RPE (Rate of Perceived Exertion) during the first half.  You should feel like you are pushing firmly on the pedals, but if your legs are burning, you are probably going too hard and will pay for it later.

In general push towards the top of the HR band on the inclines, and ease off a little to the bottom of the band on the declines.  Using this minor variation in effort is more efficient as at higher speeds, wind resistance is higher, and you are gaining less speed for every Watt of power you are producing.