Waste of money.  Buy a 6 inch caliper.

That gage covers many cartridges, and there are dozens of others that it doesn't cover.  All it tells is whether the case is trimmed to the length Lyman determined is right, and I'll guarantee it's manufactured with tolerances.

A caliper tells you the length of the case, plus how close you're getting to the desired trim length while setting up the trimmer, and then the trimmed length.

It's used for other measurements that are more important, the length of the loaded cartridge during set up of the seater die, and the case headspace when used with clamp on comparator gages.

You'll also find other uses for a caliper as you go along.