The dosage varies hugely, often more experienced vets are willing to go higher or go with dog metacam.
For example, when we brought home Ollie he had been prescribed 0.4mls dog metacam for painful deformed back legs and hips, but our vet didnt feel he was in so much pain and as he was young, just over a year old, they suggested we reduce to wean him off it and see how things went- and he now only needs a bit of metacam occasionally.
Our vet also initially prescribed 0.4mls dog metacam for Clover who came to us with a badly healed broken leg, which again as she was only 2 and improved a lot was reduced then stopped.
But more recently Clover now age 5 was prescribed 0.5 mls cat metacam twice daily for arthritis causing noticeable pain, which a second vet revised to a 0.25mls cat metacam maintenance dose twice daily once she seemed pain free.
So a huge range, and sometimes if the arthritis flares up or is progressing fast and piggy is in pain the prescribed dose should increase on the vets advice until they are pain free- but if the arthritis is fairly stable and under control a lower dose may keep piggy comfortable.
If high dose metacam isnt effective I believe tramadol or similar can provide stronger prescription pain relief, either alone or in combination with metacam.
So although this doesnt really answer the question... the vet should be guided by piggy's pain levels, and the amount of inflammation- no piggy should be in pain, but vets may be conservative to begin with if they are following the textbook dosing guide rather than their own experience based on many years of piggy practice!