No, there is no rough calculation. Insurers advise us it is a vastly complex process of underwriting based on many different algorithms, which are way to complex for you or I to understand. Alternatively, they choose to be opaque and would dread to be transparent in how they actually arrive at a premium. When challenged, they advise they will not disclose the process as this could put them at a commercial disadvantage. Yet, I cannot see how it can if they arrive at similar premiums.