The voltage tells you little about the state-of-charge. State of charge is not a simple thing to determine on NiMH. In fact, the voltage initially increases while charging, though only slightly, and near the time where it reaches full charge it actually decreases for a brief time.
It gets rougher with solar cells because the current is not consistent. You may get 200ma at first, but later in the day you have only 50mA. With the drop in current, the battery voltage will go down. Likewise, a fully charged cell that was at 1.25v under a cloud may be suddenly exposed to 200mA as the sun comes back. It may take awhile for the battery voltage to rise again.
You may have problems with overcharging, because those look like fairly small batteries. 200mA could be enough to dry them out if left on too long after fully charged. But then again this probably isn't going to be a setup used for a really long time.