NimH cell voltage is not the best way to determine state of charge - but it can be used if other means are not available.
The normal means is to charge at about C/1 rate (eg 2500 mA for a 2500 mAh battery etc) and monitor the battery voltage for a DIP in voltage at full charge.
Another OK method is to charge at about C/1 and monitor the RATE of heating (not te temperature). When the heating rate suddenly increases the cel is ~= charged.
A 3rd method is absolute temperature - not recommended unless essential. If the battery gets too hot to hold it's charged :-).
Finally, cell voltage while charging can be used. This varies with charge rate and somewhat with battery model and manufacturer so you need to take due care.
NimH cells have a fully charged voltage ON the charger of about 1.45V per cell at lowish charge rates - say C/5 and below.
The "nominal" OPERATING voltage is 1.2V/cell so 7.2V battery contains 7.2V/1.2V = 6 cells.
6 cells fully charged on charger should be ABOUT
6 x 1.45V = 8.7V, so your measured voltage is about right at 8.35V.
8.35/6 = 1.4V / cell which is OK at very low charge rates.
7.58V/6 = 1.26V/cell so too low if on the charger.
Once charged and disconnected from the charger a fully carged NimH will read about 1.3V/cell.
1.3 x 6 - 7.8V
so IF the reported voltage if 7.58V is OFF the charger it is close to OK.
If it is while charging it is too low (probably).