I don't understand the question. Slic3r should know the dimensions of your print bed and will then automatically arrange the print around the center of the bed. If it's a single object it will print at the center, if there are multiple objects they will be placed around the center. Pronterface just loads the gcode that slic3r produces and does not reposition the parts. What is an offset heat bed- is the printer's bed offset in the Z axis? Why?
Since you're a noob, I'll offer this advice about pronterface and other host software: host software is great when you are first getting the machine up and running and tuning performance, but sooner or later you're going to have a problem caused by any of a hundred things- USB comms problems, auto rebooting, autosleep, etc., that will kill your print. When that sort of thing starts happening, you can spend days trying to anticipate all the possible things that can go wrong and then play with settings in your computer to try to prevent them, but you will invariably discover new problems as your prints get bigger and take longer to complete. Failures will occur 11 hours into 12 hour prints. There are settings that can compensate for a USB cable fail/unplug, etc. As soon as you're done playing with printer performance settings, get yourself an LCD/encoder/SD card reader for the machine and print from SD cards. It is much more reliable.
Ultra MegaMax Dominator 3D printer: [
drmrehorst.blogspot.com]