While there is no document released by the CPK that eloquently lays out a plan to kill a certain percentage of the people in Cambodia, looking at death toll statistics gives a few indications.

So, starting with the roughly 8 million people who were in Cambodia when the Lon Nol regime fell, we split that into the main categories that were used to align the social hierarchy in Democratic Kampuchea: 'New People' and 'Base People'. The New People category includes 2 million urban Khmer, 600,000 rural Khmer, 400,000 Chinese (all urban), 10,000 Vietnamese and 10,000 Lao. For a total of 3,000,000. Of these it is estimated that around 870,000 perished. The 'new people' designation is all about class background and perceived 'reactionary' attitude to the revolution. Basically these people had the opportunity to 'join the cause', but they stayed in the cities due to their bourgeoise-ness (according to the Khmer Rouge). So that is 29% of New People 'perishing', not dying from outright execution mind you.

The 'Base People' group breaks into rural Khmer with 4,500,000 (majority of the population) the Muslim Chams, 250,000, rural Thai and Vietnamese 20,000 and the hill tribe minorities like the Jarai with 60,000. so of that 4,850,000 it is estimated that 790,000 died. Or 16%.

Added together the total 'perished' is 1,671,000 (this is from Kiernan's book The Pol Pot Regime - but his stats are pretty close to the ones the ECCC are using) which is 21% of the population of Cambodia. Probably about 500,000 of those are outright executions - lets call it 7%.

So, how much of the population did the Khmer Rouge intend to kill? At least 7%. How much of the deaths in Cambodia attributed to overwork, malnutrition and disease you want to deem 'intent' on behalf of the Khmer Rouge would be up for debate, it was certainly their fault but that doesn't answer your question. Remember the 'Cambodian genocide' is a slightly misleading name for the Khmer Rouge intentions during their time in power. They were not trying to kill everyone in Cambodia, if anything they needed more people to tend to the rice paddies. This is a pretty complex subject that I have touched on here, but I would say that the former urban elites, the suspicions within the Eastern Zone cadre and the former Lon Nol military, as well as the Vietnamese/Cham provide examples of classes or groups that the Khmer Rouge absolutely intended to kill.. I think they would have killed most of the Cham if they stayed in power. If I was to make an educated guess it would be like 15% of the population was probably marked for murder or was deemed to have done something that warranted execution.

This is a fairly hastily put together answer but I would be more than happy to explain some points further or answer any follow up questions, but I hope that gave you something that looks like what you what you were asking for.