While it generally works OK on single cylinder motorcycles and effectively twins also like Subaru's one must always be carefull with higher compression engines or Turbocharged ones to make sure the Chamber volume is at least consistent between cylinders or it will ping/detonate on one cylindser under the right circumstances.
Even some machine shops get that wrong so it pays to measure the actual head thickness at each end on a Subaru ie measure from head gasket surface to rocker cover gasket surface.
With the example shown while warpage is obvious it is centralised as normal with Subaru heads so the chamber volume while smaller will likely be even between cylinders,However if the same occurs with a set of V8 heads or an inline 4 then the outer cylinder can end up with a noticeably higher static compression ratio than the inner cylinders,With a high performance hi comp or turbo motor that can lead to detonation issues.
I have also seen heads from a datsun B110 (1200) that were dcifferent to the tune of 60 thou less material at one end done by some machine shop so when I measured the chamber volumes there was a range of 11cc difference between one end and the other,Just a warning of another thing to look for when doing this method or for that matter after the shop machines something for you.
A cheap method of cc heads is to use a bit of perspex and go to the local Vet and buy a graduated syringe a big one to fill with kero to actually do the measurement,Seal the perspex with grease and it needs two holes in it,One to fill through and one to bleed the air.I am lucky enough to have a proper burette setup but the above method works OK done with care.