Invoking AC21 when working remotely for employer

[nightshade]

I work full-time on H1-B for an employer in CA out of my home-office in TX (since they don't have an office in my area). When filing for PERM, we decided to use the employer's main office (in CA) as the primary work location. I understand that I'd have to move to the employer's location for some time after the I-485 is approved. I had the following questions regarding AC21 in this scenario, 

1. Can I file my I-485 while continuing to work from my home office in TX, and still avail of AC21 after 180 days of filing the I-485? 
2. Basically, is the 180 day period for AC21 only related to working for the sponsoring employer, or do I have to work specifically from the location the PERM was filed for. 
3. If AC21 requires that the employee work from the location in the PERM for 180 days, would I need to move to CA just before filing my I-485? 

Any info regarding the above would really help me decide on what the best time would be to relocate and maintain my ability to use AC21 (if I ever needed to).