The storage of chemicals in ISO tanks / containers / IBC's etc.

When asked if we are able to store a Dangerous Goods Product, I first ask them to send me their latest Safety Data Sheet, and I look at section 2.

In this example the Product is UN2651 Class6.1 PGIII 

(PG I's have a higher degree of danger than a PGII which have a higher degree of danger than PG III)

This particular product states - Specific target organ toxicity (single exposure) Category 1

So then I'll go to the COMAH Schedule 1 part 1 The Control of Major Accident Hazards Regulations 2015 (legislation.gov.uk)

Where I see that "Specific target organ toxicity (single exposure) Category 1" Lower Tier threshold of 50T

And based on the other Dangerous Goods on site I see if we exceed the COMAH  levels then either say yes or no whether we can store it.

An establishment is a lower tier establishment if the sum:

q1/QL1 + q2/QL2 + q3/QL3 + q4/QL4 + q5/QL5 + … is greater than or equal to 1,

Is this approach correct? Are there any caveats so that perhaps on tank could be 200M away from the other one? Or is it based on boundaries? These inland terminals are huge, I don't know enough about COMAH to know whether the or not there are exceptions to rules etc.. Theres rules againt Quaysides etc when unloadingcargo ships but not sure about inland terminals

If anyone knows I'de be grateful