I have a vary x one 3-9 with two mill dots below the cross hairs. they line up at 100 200 and 300 yards just fine.. It looks like the 400 yards will be very close to the bottom 4 plex expansion.. Im thinking it would be faster with a laser distance finder not haveing to fool with the knob? what say you experts..dave

You are correct, it can be faster with training/practice.

Take the rifle out and test the dots at long range.

Sometimes it is advantageous to pick one dot and sight it in at exactly "its" range.
Usually I pick the dot that corosponds to the distance I think I will be using the most.

Then I check the other dots at "their" distance.

For example you might feel it is better off to be one inch off [high or low] at 100 yards that 3 inches high or low at 300 yards.

Also:
Rarely will distances be in exactly 100 yard increments. This is where practicing at "in between" distances using hold off.

Pick one good load and practice with it.