I'd have thought the how 2 use instructions would be on the cans

 

My local garage offers the forte diesel treatment and engine flush (for £25) whenever my car goes in for servicing.

I can't imagining them phoning up anyone who takes up this offer and saying, sorry we can't supply the diesel treatment because you have a full tank of fuel.

That aside, with other fuel treatments (eg Redex) it suggests adding it to a full tank of fuel.

In short, I can't see it makes any difference when you use it, but using it on a nearly empty tank of fuel will mean its more concentrated.

 

Further info, from a Google search.

How to use Forte diesel injector cleaner:

1. Simply add the content of the bottle to your fuel tank and then add approximately 40 litres of Diesel to your tank.

 

🙈 i take it this is your first car? anyway i put redex in mine neat never had a problem with engine or emissions