Seems like the key is just multiply 2 dice, or 3 for the real big numbers, to get in the general ballpark, then add/subtract to zero in on a key number. If you can't find a way to get close with 3 multiplied dice, then use 2 multiplied dice as an add/subtract. Try to hit the key number the quickest most direct way you can, then try to find a way to make any remainder dice cancel out.