Piercing provides flat penetration. In the Live version a more accurate tooltip would be something along the lines of "Increase physical penetration by x% of 120*Level (7920 at V16)". This has been made more clear in the Thieves Guild update where it will read "Increase Physical Penetration by x". In the Live version you are probably best served putting 100 into Mighty and 66 in Precise Strikes and 1 in Piercing. The 1 in Piercing is because 66 and 67 points in Precise Strikes works the same way due to rounding. This advice is generic advice given without knowing your class, rotation or content you want to clear. In the Thieves Guild update you can try the following spreadsheet to help in your CP distribution. It is designed for magicka builds but can be applied to stamina builds if you are happy with the assumption that all of your damage is physical based. Simply make the obvious substitutions

https://docs.google.com/spreadsheets/d/1Zp9v1Vp4Z7X6zfDfcxTwyAnejv-tEC5LujbXYBiVMDk/edit#gid=599371859.