Yes, you can. However, because the bars display is a graphical representation of signal strength and not a true signal strength reading, the accuracy of the antenna placement may be limited.
weBoost recommends that you locate and aim your Yagi antenna based on your phone's actual signal strength reading. The signal strength reading is typically displayed in decibels (dBm).
The reading is often displayed as a negative number, like -91 dBm, for example. But on some phones the - sign is omitted, so the reading displays as 91 dBm instead.
Regardless, a signal strength reading CLOSER TO ZERO is stronger. For example, a reading of -86 dBm (or 86 dBm) is stronger than -91 dBm (or 91 dBm).
To find the signal strength reading for your Android phone, click here.
To find the signal strength reading for your iPhone, click here.
You can also consult the user guide that came with your phone, or use a search engine to find the term "Signal Strength Reading' or 'Field Test Mode.' Or contact your phone manufacturer's customer service department.