The perceived wisdom is major diameter minus pitch equals drill size. The wisdom holds true irrespective of how many leads the thread has.
Yours both have a 0.8mm pitch, therefore:
14 - 0.8 = 13.2
12 - 0.8 = 11.2
Using a drill that is too small will be less successful than using one that is too large. A 13 or an 11 will result in torn fibers whereas a 13.5 or an 11.5 will result in a successfull cut but a looser fit. If you have drill bits that are near to size but not smaller, I suggest you experiment on some scrap material and see if you are happy with the results. If you feel that the result is too sloppy or not secure enough, that is the time to invest in the correct drill sizes. There is sometimes merit in having a looser fitting thread on the cap. Tight fit equals abrasion, equals wear and tear, but the decision needs to be assessed according to the material you are working with.
