how do I calculate the CO2 emissions per turnover of my company

Emissions divided by revenue, no new measurement

The CO2 intensity that customers ask for is a ratio: total emissions in tonnes CO2 equivalent, divided by revenue in millions of euro. There is no new measurement involved — you reuse the scope 1 and scope 2 figures you already have, and include scope 3 if your customer explicitly requests it. If scope 3 is not mentioned in the question, leave it out of the numerator; most customer questionnaires in the VSME basic module ask for only scope 1 and 2 for this metric.

The practical question is which revenue you use. The standard is the revenue from the most recent annual accounts, for the same year as your emissions figures. If you have multiple locations or report at group level, use the revenue from the same scope as your emissions — otherwise the ratio will not be correct. If you have not yet separately documented scope 1 or scope 2, that is the first step: see the direct emissions of greenhouse gases and indirect emissions from purchased energy for what exactly falls under those. Without those two basic figures you cannot calculate the intensity, and then 'not yet available' is the honest answer instead of an estimate.

Source: VSME basic module and the scope classification of the GHG Protocol

The metric itself — emissions per euro of revenue — comes from the basic module of the EFRAG VSME standard, which expresses environmental figures as an intensity measure so that customers can compare companies of different sizes. The subdivision into scope 1, 2 and 3 that you need to compile the numerator comes from the GHG Protocol; that is also the source for the question of whether you include scope 3 in this specific metric. If you are unsure whether a customer wants scope 3 in this particular metric, look at the wording of the question itself — that is what this explanation of scope 1, 2 and 3 together goes on to.