Only An Idea?
I know it's a bother, but why not get a couple of five gallon containers or perhaps smaller
containers of straight B100 and add the correct portions to whatever you figure would
be correct to maintain 5 percent without of course going over five percent?
Add it as you are filling with straight ULSD so it will be mixed properly.
Messy perhaps, but a way to be sure that what you wind up
with is five percent (B5) and not something more or less.
i e . . One gallons is 128 fluid ounces, so five percent would be 6.4 ounces, correct?
When filling and you figure it will take ten and one half gallons to fill the tank,
add two quarts as you are filling and you'd have a five percent ratio (B5),
and you'd know that you have exactly five percent, no more no less!
Perhaps you'd even come out ahead money wise?
Only an idea mind you.
HTHs