Does anyone have any knowledge regarding values?
know it's not good practice to use only one resistor to limit the current going to parallel LEDs, but it works and the resistance is enough to prevent a single LED blowing.
Assuming an hfe of 100, you need at least 1/100th of the LED current into the transistor base. A reasonable rule-of-thumb is 1/50th to 1/20th (2%-5%) to make sure the transistor turns all the way on ;(saturates)
Do you know why it's bad practice? ...The current (around 25mA in your case) gets divided among the parallel LEDs, but it does not necessarily get divided evenly, depending on variations in LED characteristics. That means some LEDs may be brighter than others.
Two LEDs in parallel each drawing 6.25mA will be the same brightness as 2 LEDs in series each sharing the same 6.25mA.Drawing twice the current will certainly age the battery faster.