for a couple reasons
if you are makeing an ackley the point of impact will be very diffrent from the parent case so you will have to sight your rifle in twice
less wear on your barrel with just a little fire and no bullet
whats the point of makeing a unique casing if your just going to shoot regular bullets anyway
I have fire-formed at least a dozen different cartridges over the last 40 years and never used a reduced load, or a load without a bullet.
Just form the brass correctly in your dies so you don't have excessive headspace, load and go to the range. When you get there you will discover that the point of impact is virtually the same as subsequent loads in fireformed brass. No fuss, no muss, no extra barrel wear.
I have even hunted with fireforming loads and not lost a moments sleep or game animal doing so.
Ted