Tables :: Calculated Fields From Two Tables / Based On Relationship In Third Table

I have two tables of data, each relating to three business branches (branches A, B and C).

Table 1 shows the expenditure of each branch (by fuel, premises and wages).

Table 2 shows a number of units for each branch (mileage, floorspace and sales).

What I would like to do is calculate unit costs, based on the expenditure in Table 1, divided by a relevant unit in Table 2. The catch is that I want to have a third table which allows the user to specify which expenditure (from Table 1) is combined with which unit (from Table 2) to generate the calculated unit costs. I've been able to do this in Excel, and have attached an example. I've also attached an incomplete Access version with the first two tables. Given the complexity of my actual data, I feel this could be better handled in Access than Excel.