"I am sorry but you got this part is (mostly) wrong. Just stick to 300 ppi in your final print size. Please don't tell people anything else.
Note: yes, there are exceptions to the "300 ppi/dpi" rule."
-- I'll agree that 300 PPI is a good rule of thumb. You're not really going to go wrong with that. You'll also note that this dovetails with much of the advice I've been giving (for instance, the advice that you don't need to make your PPI equal to the DPI of your inkjet printer.) The rest of what I wrote is the result of many years...*ahem* ...I should really just stop and agree to disagree. I'll simply say that I'm not making things up. I've spent hundreds of hours looking closely at printouts from many different kinds of printers over many years, through a loupe, and doing multiple resolution and color gamut tests with both client files and test files, as well as having read many books on these subjects. I'm not going to question your qualifications, but simply clarify my own.
"Telling people that it's ok to print stuff in less than 300 dpi/ppi is just wrong."
--Strictly speaking, it's just not wrong. The PPI you use is directly related to, and limited by, the DPI of the output device. However, as agreed above, 300 PPI is a perfectly fine rule of thumb that will not steer you wrong most of the time.
"And you should also check out what postscript is and how it works. Before you tell other how you think it works."
--PostScript is a page description language, developed primarily to transmit object-oriented content to printers. I haven't personally hand-coded PostScript, but I've known people whose job it was to hand-code it, and I've looked at the code many times. It's an industry standard way of describing vector graphics. It's also very flexible, and it can encapsulate raster data in addition to vector data. In our graphic design workflow 10 or 15 years ago, we used lots of Photoshop .EPS files because they were the only raster format at the time that was PostScript Level 2 compliant, and we were working in a color-managed environment where we needed our RIPs to do intelligent color gamut mapping.
Again, I won't claim to be an *expert* about PostScript, but I do know quite a bit about it and have used it for many years. I'm curious what you thought was incorrect about what I'd said?