MAC is the "mean aerodynamic chord". Basically the average chord. So if you have a tapered, swpet wing, you average it out and take 10-15% of that.

Here's a short hand way to do it (assuming constant taper and sweep, no funky curved surfaces). Take the tip and center chord measurements and average them. So a 10" root and 6" tip would give you 8". Then find the location on the wing that has an 8" chord line. That's your MAC. If you take that point on both the left and right wing. Then go back from the leading edge 10-15% and make a mark. That's your CG. Connect the marks, and you can see where the CG is at the center of the wing for easier balancing.

The proper amount of reflex will be determined experimentally. After all, we are talking about elevons. When you get it trimmed for level flight, you have the right amount of reflex.

It's very common for the first few flights of newly designed wings to be really really REALLY short ones. I've seen the instant power dive to the ground, and the immediate loop of death. The reason is that the reflex was wrong, making the pitch trim way off. Mix that with a rearward CG, and it can be really crazy. Also, even properly tuned wings tend to be really sensitive to elevator commands, so you really don't need much throw in that department.