To calculate the IV of an installation it's not so much the formula you need but the tables containing the IV of each pipe size/material and the meters, of course you still need the formula but it's easy:
Split the installation into pipe lengths for each diameter then multiply the running length by the volume per metre from the table eg (all volumes are per linear m for copper)
15mm is 0.00014
22mm is 0.00032
28mm is 0.00054
35mm is 0.00084
Once you have added all the pipe lengths up you then multiply the total by 1.1 (to add 10% which is an allowance for fittings! it's the same allowance for every install)
You then add the IV of the specific meter (info taken from another table)
Eg U16 is 0.025
The total is the IV of the installation and needs to be under 0.035m3 to be worked on by a domestic engineer
The next step would be to calculate the PV (purge volume) this is simply done by calculating the IV as above then multiplying the answer by 1.5