By looking at a number of 'reference installations' considered to be typical of each technology and size.
Chapter 6 of the government's RHI announcement says
The tariffs vary depending on technology and size in order to match actual cost levels as closely as possible. Nevertheless, even within the range of installations covered by any one tariff, the level of support that each installation actually needs will still differ, sometimes significantly. Differences arise, for example, depending on whether the building is a rural or urban building, or whether the building would use oil, coal or gas if it did not switch to renewable heat (e.g. if the alternative to renewable heat is oil or coal, which are relatively more expensive than some other fossil fuels, it would need less support in order to make the switch to renewable heat attractive than if the alternative is gas, which is cheaper). In order to find a compromise between these cost differences, we have calculated each tariff on the basis of a so-called ‘reference installation’, which represents (in most cases) roughly the mid-point of all the installations covered by the tariff in question