You can start an argument with that question. If you own the .284 reamer you use, there is perhaps a better way. Have a custom die made for you. In the States, Jim Carstensen does this, as does Niel Jones. With Carstensen, you send the reamer and a couple fired cases, I'm not sure Jones even needs the reamer. It then becomes their business to make a die that results in about .001 clearance after sizing.
The cost of a die from Carstensen is less than a FL reamer plus paying for a blank & hardening the die, so even if you can make your own, you'd be out more. Jim modifies an existing die. I believe the Jones die costs a bit more, but it uses a different type of bushings for the neck & shoulder, & some people think the design is better. I've used Carstensen dies, they are first rate. No experience with a Jones, but his reputation is very good.
I believe Hornady will also make a custom die from your fired cases for significantly less money. Too, they don't need your chambering reamer to make the die, but since it is made from fired cases, it will still be tailored to that one reamer. Some have claimed these dies are excellent, some were less satisfied. It could be that Hornady just has a larger "audience," & there is an old adage about pleasing everybody.
The problem with specifying a reamer is, "how much does the brass spring back after sizing? And that depends on a number of factors, your load, your brass, how long use (have to) use cases, etc. Rule of thumb is about .003 smaller for a sizing die, but you'll find people who want .0025 at the shoulder & .002 at the base, or some who go the other way.
With having a die itself made, you pass that burden to the die maker, you get results, not theory.
