From a strictly Math point of view, that sounds like you want the log base 2 (log2) of the value. So, the log2 of 64, for example, is 6, which means the 6th bit (starting at bit 0) is the MSB. 64, is, of course, 1_000_000 in binary. And you can get that by doing log($value)/log(2):

  DB<4> x ( log 64 ) / ( log 2 )
0  6
  DB<5>
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