There is no general answer to your question.
An ideal intraocular pressure range for a glaucoma patient is very individual. It's usually based on a judgement call by one's glaucoma specialist. The rule of thumb is that the more advanced your glaucoma, the lower the target IOP will need to be to slow or prevent more damage. It also depends on the type of glaucoma and many other factors.
Another rule of thumb in glaucoma might be "lower is better." However, achieving a lower IOP has a cost. Generally that cost is more medication (with the risk of more side effects and more expense) or surgery (or additional/multiple surgeries). Surgery should not be performed unless it is definitely needed. Hence, a glaucoma specialist weighs the costs versus the benefits in setting the target IOP for a patient.
Some glaucoma patients may have a target IOP as low as 5 mmHg. (I suppose it is possible that the target IOP could go as low as 3, but I have personally not seen that.) Other glaucoma patients may have a target of above 20 mmHg. For example, if one was diagnosed with an IOP of 40 and bringing it down to 21 resulted in no further glaucoma progression, then one's glaucoma specialist would likely conclude that 21 mmHg is a suitable target IOP for this patient.
To give you some reference, the normal intraocular pressure range seen in healthy eyes is about 12 to 21, with an average of around 15 mmHg.
In regard to your specific tonometer, the Reichert 7CR, you are presented with two IOP values: IOPg and IOPcc. IOPg is correlated with Goldmann applanation tonometry, which is the most widely used value in the scientific literature to date. See this article for more:
Is Goldmann Applanation Tonometer (GAT) the Gold Standard? | Ask FitEyes
IOPcc is a corneal compensated value that is closer to your true IOP. Obviously, it should be preferable to use IOPcc. As the article above explains, all glaucoma specialists should be using more modern tonometers like the Reichert 7CR, ORA, Pascal DCT, etc. However, there are conversations in which you may want to reference IOPg so you are "speaking the same language" as someone else who is referring to Goldmann applanation tonometer reference ranges.