Does eye pressure in glaucoma usually need to be lower than the target pressure in order to prevent damage?

Not necessarily: Usually shoot low because pressure fluctuates during day, so it helps keep it in "good" range all day. Also no benefit to high-normal pressures.

Not usually: The 'target' pressure is set at the level which should be protective. If your treatment attains that pressure you should be fine. Getting it below that probably offers no further protection although you might find that to be a protective cushion. Best to follow the advice of your treating ophthalmologist (optometrists should not be trying to treat glaucoma).

Target IOP: The target iop is the pressure your doctor has selected to try and maintain the intra ocular pressure (iop) below. Once achieved the eye is monitored to ensure no more damage is occurring. If testing and monitoring demonstrates further damage, then the target iop must be lower, if no damage is occurring then it is adequate. The goal is to lower it below a point where further damage does not occ.

Yes.: The definition of the target pressure in glaucoma is the pressure that is necessary to achieve to prevent progression of the disease in an individual. In general the target pressure in mild glaucoma is the high teens, in moderate glaucoma the mid teens and in advanced glaucoma the low teens or even single digits. The target pressure must be individualized depending on the patient case.