Add transparency on a spot color for printing

The printer's process of your file and the actual print process doesn't matter much if your file is prepared properly it can be processed in an optimal way.  

I recommend you find your color, then specify it as a solid Spot color in Lab. That will provide the printer with a named color and its Lab values.  If your job prints digitally, and your color is too bright, it will not print to match, because the press gamut doesn't encompass your color.  The advantage of specifying this way is that the special color can be added to the press and proofing RIPs so they will be able to adjust that color individually if needed.    

If the color is to print as a spot color then you can ask your printer for a drawdown of the ink color you have specified.  Then they will use that color on the press.   

The best practice is to specify spot colors as Spot in your files at 100% if possible.  If not so long as the color is set up as a spot color in your file as a Lab color it will go through all of today's rips very well.  

The match may be disappointing if you're specifying a color that is out of gamut for a print process, and your print job is CMYK.   

That's not best practice. Instead, play with a "tint" of a character - this is a percentage or proportion of the ink. This should be able to get the same results without the risk of transparency.  You can also use tints to get different colours without paying for multiple inks.

So you reduced the opacity? In that case that would need to use halftone screening in order to make it lighter. Your design would be printed in flexo? In that case halfone screening might or might not produce the exact color you want. You can do halftones, but only to a limited amount and it might turn out different. Since you seem to have no experience with out, I would consult with a flexo printing pro for this job.