To calculate an "acceptable" dose level of genotoxic carcinogens, two parameters must be known. First the slope of the linear dose-response line. This is usally known as the "slope factor"; these can be found in the scientific opinions and in various databases (e.g. IRIS). Regarding the IRIS database a background document is presented where the extrapolation is described in more detail. The second factor is the level of cancer risk to be accepted in the population due to exposure of chemicals in food and feed. For that level various levels are known, leading to some confusion by laypeople, but most experts use a risk level of 1 case in a population of one million (1 in 1,000,000) exposed persons per year, being equal to 1 case in ten thousand persons (1 in 10,000) after lifetime exposure. For comparison: the global background cancer risk for all cancers is estimated in 2018 to be about 20 per ten thousand in that year.
The calculation of the risk using a slope factor is rather straightforward. The risk is equal to the intake times the slope factor. One should define the lifetime exposure to the chemical of interest and express it in mg per kg bodyweight per day. The unit of the slope factor is 1/(mg per kg bodyweight per day). The multiplicaton of exposure times slope factor is therefore unitless. The results is usually a value below 1, e.g. 0.001. The latter value is to be interpreted as 1 in 1000 (1/0.001). Situations where the result is greater than 0.0001 are to be considered "unsafe".
To calculate a cancer risk some online tools can be found on the internet, e.g. the carcinogen slope factor calculator.