A319:
(2xGW)+85 = green dot
Or to be more precise:
(2xGW)+85+ correction (1kt per 1000ft above FL200) = Green dot
So when in cruise you can deduce your GW (well at least your FAC calculated GW) by taking Green dot from the PFD. Substracting a correction of 1kt per 1000 ft above FL200. Substracting 85. And then divide by 2.
(Green dot (PFD) - correction (1kt per 1000ft above FL200) - 85) / 2 = GW (FAC)
You can compare this GW vs. GW on the status display or FUEL PRED page to assess FAC accuracy and/or loadsheet accuracy.
CI=0 will give you best rate of climb speed indeed. (ref: Getting to grips with a/c perfomance (airbus)) There is no rule of thumb but the SEVERE TURBULENCE speeds are a good approximation. (ref. FCOM)