Oldsock,
What was your reasoning for the 0.15 oz of oak cubes per gallon? Is this 0.15 oz dry or weighed after soaking? Do you assume they are boiled to reduce the oak strength before use?
I ask because I had found detailed calculations on another site that come up with 57 square inches of barrel surface area per gallon of beer averaged with a standard 53 gallon wine barrel. The oak cubes I have are 1/2x1/2x3/8 inches. This equates to 2.5 square inches per cube. That would be 22.8 cubes per gallon of beer. 23 cubes weigh 0.6 oz dry. That is four times what you recommend.
I'm sure Oldsock will give his own answer soon enough, but I assume that his recommendations were based on his own experience of the oak-flavours added by fresh (but boiled) cubes, and what he prefers.
I'm curious as to why one would want to have the same surface area as a used barrel? Obviously you're not getting micro-oxygenation from the cubes. And its not at all obvious that having the same surface area of cubes as in a barrel will result in the same flavour contribution if we're comparing fresh (but boiled/steepd) cubes to old wine barrels.
Oak flavours are going to be more muted because the barrel will already have been used to age several wines, which must surely strip more of the oak flavour than boiling the cubes, and even than steeping them in wine for a few months. In fact, in the Belgian tradition (and English) at least, brewers are using old barrels because they don't want too much flavour from the oak.