The big secret? No one measures pack volume the same way under the same conditions so comparisons are vauge at best.
Your best bet is to call the manufacturer of the rig you plan to put it in, and ask them if it will fit. Back that up with testimonials from jumpers who have actually packed that canopy in that size container (you can get that here in the forums), and you will have made an informed decision.
I assume you are still wondering whether your Xaos will fit in a Voodoo 00. The short answer is yes, just like I told you in the last thread
A dude at my DZ has a Velocity 90 in a Talon FS 00, and it fits great. Your Xaos 21 88 should have a very similar pack volume to his Velocity. As for the container, according to RIs website, the 00 FS and 00 V2 have the same main pack volume of 275.