Setting ppi on export for LARGE prints

So I have done some research and everyone says to set their ppi to 300 when exporting from lightroom for print. I am looking to blow up some of my 20mp photos on a metal print, and the printer is capable of printing at 1,200 dpi. Should I set my export size to 1200 ppi if I want max quality on my print? Any other advice to export for print, other than the basic stuff like setting quality to 100 and saving it as a jpeg?

One thing to note is that the printer being capable of 1200 dpi probably does NOT mean it can reproduce detail at that level. It usually means that it can spray ink droplets at that size (dots/inch). Generally such a printer can produce greyscale/color detail at much lower resolution such as something like 300 pixels/inch. The old rule used to be 2.5 times lower than the screen resolution which means this printer might be able to show detail at around 480 ppi if pressed, but you probably don't have enough resolution to get there in your source file anyway, so aim for 300 ppi. This is due to the fact of how colors are made on such printers by higher and lower density of the ink droplets. Very generally, the way to get the absolute best quality is to export at around 300 ppi at the size in inches or cm (as Jim demonstrates above) that you want to print and (EXTREMELY IMPORTANT) to use a certain degree of output sharpening. Usually "standard" is enough but make sure to specify the type of paper. For metal prints glossy is usually right. Interestingly, even if your image is higher resolution than this, you actually get better prints with more visual acuity by scaling to 300 ppi and using good output sharpening than by sending the full resolution file. This is due to the inherent softening caused by the printing process needing to be counteracted at the right resolution which is usually right around 300 ppi. If you try to do it at 600 ppi it will not do anything as the printer can't actually display the effect.

Second part of good prints is to have control over the color. This means that you should have your display well calibrated and that you need to have a valid color profile for the printer/paper combination. If the print service doesn't supply color profiles, make sure to know what color space you can send files in. If they can do adobeRGB files that would be fine but even if you can only send sRGB that might still be OK as long as the color in your images is not too saturated. Softproofing can help here if you know how that works (many tutorials on the web).

Last but not least. There is no point in using quality 100. In your final print, you will not be able to distinguish between quality around 85 and quality 100 (I've tested this extensively) not even with a magnifying glass, but your file will be many times larger if you use quality 100. Likewise, you cannot see the difference between a jpeg and a tiff file if you do everything right as described above.

Hope this helps

Setting the PPI won't do anything to change the quality of the image that you export. For example, if you take a 24 MP Image that is 4000 x 6000 pixels and export it at 72 PPI or 300 PPI or 1200 PPI it will still be a 4000 x 6000 pixel image. And just because you change that setting, quality of the image isn't going to change one little bit. You simply need to do the calculations to ensure that you have enough pixels to produce the quality of image that you are looking for. If you want an image that prints 16 x 20" at 300 PPI it needs to have the following:

16 (inches)@300 PPI = 4800 pixels

20 (inches)@300 PPI = 6000 pixels

So to print a 16 x 20" print that has 300 pixels per inch the image needs to measure 4800 x 6000 pixels. And it doesn't matter what the PPI setting of the image is.