As a general rule, you should add your Diesel Kleen, Biocide, and lubricity additives before adding fuel to the tanks. Estimate how much you will add based on hours run since your last fill up then treat that quantity of fuel. This way, even if life or Biden, gets in the way, everything in the tanks is properly treated.
Another factoid about fuel, is if you keep your tanks topped off then you likely spend less money than if you just add some fuel for ,say, Saturdays outing, plus if you follow the previous paragraph, all your fuel is properly treated, all the time.
understood, so follow up question to that (maybe this is more a question for the treatment manufacturers)
I treat just as you describe at every fill up using the maintenance quantity of each additive. So, that 10 month old fuel was treated when it went in. Do those additives diminish over time requiring a boost, when the fuel isn’t burned?