about onscreentext alignment, accurate center, how?

i found onscreentext are not accurately aligned to center even if set to textnode.Acenter. in fact, the texts always bias a little to the right, no matter what font i load. sometime i wish to use text as a cursor, so position must be accurate. how can i calculate the pos offset? i use pageSize 32 and pixelPerUnit 30, for example.
also , accurate verticle alignment is hard to achieve.