it has been a while since i have been on here.
when pring, it is your unconscious mind that is doing the work, the chant is designed to keep your conscious mind off the current task, therefore i don't really think thinking other thoughts will be that bad. As long as you keep the state/relaxation whilst pring. As for the "i wonder if i am doing this right, is this going to work?" i think those may have a negative effect. Just relax. It is very hard to do the pr step wrong so long as you open your vision, relax and don't hard focus on the words.
Paul c