If you are concerned about the CPU use, consider this:

panning value x = [-1..+1]

L_gain = (1 - x) * (0.7 + 0.2 * x)
R_gain = (1 + x) * (0.7 - 0.2 * x)

which will be -3.1 dB at the center. You can modify this
modyfying the 0.7 and 0.2 constants, for example

L_gain = (1 - x) * (0.65 + 0.15 * x)
R_gain = (1 + x) * (0.65 - 0.15 * x)

will produce -3.75 dB, and

L_gain = (1 - x) * (0.6 + 0.1 * x)
R_gain = (1 + x) * (0.6 - 0.1 * x)

will produce -4.4 dB, etc.