Centralise fields after adding

i have successfully impletemented the plugin. and i have just added a single field to the checkout page.

following the guides set out on the paid memberships pro website i have slightly edited the check out page (just changed it so that the table is in the middle of the page using a <td style=”width:50%”>, not the nicest way but it works..

I am trying to figure out how to do this to the fields added by the register helper but i am having very little success in figuring out what needs changing!

Any help would be appreciated.