I have generated a excel spreadsheet with tables and graphs of pi0 decay kinematics.
Get spreadsheet here.
I calculate quantities for pi0's from .5-50GeV energy.
The first table gives gamma and beta which are the normal relativistic quantitites by those names. Phi_gg minimum is the minimum opening angle between the two gammas and occurs when the gammas equally share the energy.
The mass of the pion is a parameter below the first table, so if you want to see what happens for etas just change this mass.
Next I assume a threshold for detecting the lower energy of the two gammas. In the spread sheet this .5 GeV and probably isn't much lower than that. This also is a parameter you can change. With this threshold set one can calculate the angle of the two gammas for the limiting case of one gamma at the threshold energy. This is theta1 max and theta 2 max. Their sum is then the maximum opening angle Phigg. Angles are converted into distances by assuming 300cm flight path. Note that observable decays occur within a band of 1-2 deg Phigg_obs=phigg_max-phigg_min.
Below there are tables of opening angles, gamma energies, energy sharing z_gg=abs(E1-E2)/Etot etc. There is also the number of decays per dphi.
There are plots of many quantities on a separate sheet.
Observations
If you consider the same pT we are looking at a much different pi0 decay geometry at eta=2 than eta=1.
As mentioned above there is actually a fairly narrow band around a seed gamma that you expect to find the other if you start from the highest energy and work your way down. Allowing for etas opens the band considerably.