Digital BW, The Print

Just interested to know what value you guys take to be absolute mid 
grey. The good old 21 step wedge in greyscale mode gives a midpoint 
value of 50% black. No surprises there but convert it to RGB values and 
it is 148,148,148 and not 128 (the midpoint between 0 and 255).
OK I know I'm simple so it should take a simple answer.

Pete.