C:70–73 M:0 Y:100 K:0

Depends on the paper you use I guess. It seems you’re using an uncoated stock, so I’d recommend something along the lines of Skye Uncoated Brilliant White, or a similar bright white stock.

Thanks Conor, but when I have my Pantone color book CMYK 70-0-100-0 and PMS 368U next to each other, the CMYK is a lot darker, it's more like PMS 369U...

Agreed. My Pantone 368 U is not the same as you have within your CMYK values.
My PMS 368U is @ C:57 M:0 Y:100 K:0

--
Rez +

*Edit* Dont know why I have said 'agreed...'

--
Rez +

You will have to concede at some point that it won’t ever match exactly… it’s definitely in that region of cyan 67%–73%. But like I said before, it depends on the paper you choose… and the quality of the printer.

Go for the best paper match, not what the computer converts it to.

If you’re really having trouble with a critical color, and the budget isn’t too tight, try getting the printer to work it out for you. They’re the ones who know the presses and inks best.

Pantone spot colors are built from special high chroma inks. Very few of the spot colors switch to cmyk without a loss of brilliance.

Here is what I got in Photoshop Standard C63 M3 Y91 K0 LAB Coordinates L65 A-44 B45, then I made CMYK 0000 then I re-inserted LAB Coordinates and got this for CMYK C63 M4 Y92 K0.

The Truth shall set you free

In Freehand MX and Illustrator CS, InDesign CS and Quark 6.5 C57 M0 Y100 0K, Illustrator CS. This recipe is lighter than the Photoshop conversion.
The Truth shall set you free