6! / 5! = 6, which is way too big. So, instead try X=15. The leading digits of 16 is still too big, so go to X=105. Repeat this process until you get a sufficiently small change in the leading digits between adjacent numbers - 10,005 barely works, so might as well go to 100,005 instead and be sure we're hitting every number.

Now the problem is that you might not have enough small increments you can make. You can always replace X=100,005 with X=1,000,0050 - multiply by ten, and the leading digits behave the same on multiplication, but you have ten times as many numbers you can multiply by.

You basically can divvy up things in as many fine increments as you want, so you can always find some number that does things properly.