Custom certificate (plugin)

I'm trying to right align various certificate details closer to the middle of
the generated pdf but the setrightmargin function seems not to be affecting anything.

cert_printtext(100, 230, 'R', 'Times', '', 18, "Text here");

The above will print to the right...but is there a way to distance it from the
right of the page? The first attribute will only shift farther to the left and I've already tried

cert_printtext(-100, 230, 'R', 'Times', '', 18, "Text here";)

Anybody any ideas?

Thanks

F