Great! I'm going to put together a part 3 summation video and I want to think about the next step/upgrade. I do want the machine stiff enough to mill aluminum, but maybe I keep this machine for wood an other materials and start from scratch, save my pennies and build a stout little milling machine.
In turn:
1. There was no before data, unfortunately. If I had to guess, I'd say that my modifications both to the Z axis: steel V-track, bearings and stainless channel, and to the X axis: steel channel, added very little rigidity where it was needed, in the middle of the X beam.

2. OK, so v-track is probably fine to stick with if I try an X beam upgrade to steel.

3. Should I use: http://www.mycncuk.com/threads/2214-...ess-calculator URL for the calculator and the general mycncuk.com URL in general elsewhere? I'm happy to drive traffic here if I can.

4. I entirely understand that your time is limited and I appreciate that you've already helped me so much! I can't believe you ran my numbers, that's so kind.

5. What you're describing is actually what I did, I think. I applied say 20lbs to the scale by pushing the router collet down, which amount to upward force. Then I did the same 20lbs for 7 points all over the machine. That is what the data represents. Maybe I'm misunderstanding.

I do have more questions about the next step, steel. But, I'll start a new thread so all can benefit.
Thank you again, routercnc!