I am withdrawing $150K from my 401K in April. Can I estimate quarterly payment on that amount only, or must I combine it with my normal income?

I am withdrawing $150K from my 401K in April.  I am 59 1/2 so no penalty.

If I treat the  $150K separate for quarterly payment purposes, there is substantial savings over combining it with my estimated earnings from my job. 

Is it legitimate to treat the taxes this way?