Models again. If you guess at how many herbivorous dinos there were, and what their total mass was, if you knew what they ate, if you knew what the composition of what they ate was, and assumed that the same intestinal flora we have now existed then and then guessed at the overall dino metabolism including digestive enzynmes, you can derive a number that models methane emission.
There is still discussion of whether the dinos were homeothermic or not.
So that’s a lot of ifs, a lot of assumptions, leading to a number, no more. Change an assumption, change the final number.
Don’t forget a comparison of the same plant material decomposing on the ground versus digesting.