Doing a G28A0 will send your machine to “HOME” position. Home position is not always your reference position. Which is what I think you are looking for. Sometimes I set up my machining centers to have home position different from machine origin (where your switches will be). You need to give it a G53 in the same command. G28G53A0 should get you there.
Not familiar with your machine. What kind of control do you have on it?
Stevo