Biocide when topping off

[AlwaysLearing]

I suspect I'm overthinking this, but I'm curious what others do when adding biocide while topping off their fuel tanks.

My 350 has a 39-gallon tank, and it's currently about 5/8 full. I'm not low on fuel, but I'll likely be at a fuel dock this weekend, so I figure I might as well top it off for the rest of the season.

The previous owner used Biobor JF, so I plan to stick with it. My question is about the "maintenance dose".

According to Biobor:

"The standard maintenance ratio for Biobor JF is 1 ounce per 80 gallons of fuel."

Google searches seem to give conflicting advice. When topping off, do you:

1. Dose for the full 39-gallon tank every time you top off?

In my case, that's about 0.5 fl oz, which is easy to measure. However, if you add that amount every time you top off, wouldn't the concentration gradually exceed the recommended level?

or

2. Dose only for the amount of fuel you're adding?

If my tank is 5/8 full, I'd only be adding about 14.4 gallons, which works out to roughly 0.18 fl oz of Biobor. That seems very difficult to measure accurately at the fuel dock.

It's further complicated by Biobor's recommendation to add the biocide before fueling. I'd have to estimate how much fuel I'll actually be adding—and we all know fuel gauges aren't exactly precision instruments.

How do you handle this in practice?

Thanks in advance for your time.

[wolfe10]

If you do want to "over-think" it:

Estimate gallons of fuel you will need.

Add appropriate amount of Biobor JF for a MAINTENANCE DOSE.

Add fuel. Stop when the nozzle clicks off the first time (almost but not full).

If you took less fuel than expected, you will have a Biobor JF concentration between maintenance and shock.

If you took more fuel than expected, add more Biobor Jf to bring to maintenance dose.

Top off tank with diesel to wash the added Biobor JF into the tank.