A special, tricky case of Less / Fewer

I understand the basic difference between less and fewer: you use "less" for things that aren't counted (There is less water in the pool than yesterday; I feel less hungry now than before).  You use "fewer" for things that are counted (There are fewer than eight cars in my garage; There are fewer jellybeans in that jar than you think).

What if what you're describing IS countable but you don't mean a full unit less?  For example.... if I said, "There are fewer than two gallons of water in that jug" this suggests that there are 0 or 1 gallons.  But what if the amount is 1.9 gallons?  There aren't really "fewer" than 2 gallons, since rounded off 1.9 is 2 anyway, but there Is "less" than 2 gallons in there... right?  The same can go with something like time.  People say "there is less than one minute left" even though minutes can be counted.  So, should I say "There are less than two minutes left" if, say, 1 minute and 57 seconds is left?  Or am I just crazy?

Thanks for your help.

I think measurments trigger "less", because you're usually thinking of the scale and not of the countable units.

For example, if you ask me: "How long did it take you to read that book?" I could answer: "Less than a week," or "Less than seven days," and I'd have given you the same information. In such a context, weeks and days are countable, but they're not counted; they're used to express an estimated period (which is not countable).

Of course, I could be led to think of the units of measurment rather than the period. If you'd ask me, for example, "How many days did it take you to read that book?" the "how many" might trigger: "Fewer than seven" instead.