It seems a PDF doesn’t link automatically, so for convenience his reasoning is below. While it seems strange to boldly claim to come up with a precise modern quantification, I assume his intention is not so much historical accuracy as meeting the need to find a reasonable measure by which members of the Sangha can be cleared or removed without lingering doubt.
The value of the object
As stated above, any case of stealing counts as an offense, but the gravity of the offense is determined by the value of the object. This is the point of the phrase in the rule reading, “just as when there is the taking of what is not given, kings… would banish him, saying… ‘You are a thief.’” In other words, for a theft to entail a pārājika it must be a criminal case, which in the time of the Buddha meant that the goods involved were worth at least five māsakas, a unit of money used at the time. Goods valued collectively at more than one māsaka but less than five are grounds for a thullaccaya; goods valued collectively at one māsaka or less, grounds for a dukkaṭa. As the Commentary notes, the value of the articles is determined by the price they would have fetched at the time and place of the theft. As stated above, in the case of smuggling the Vibhaṅga measures the value of the object, for the purpose of this rule, as the duty owed on it, not the value of the object itself.
This leaves us with the question of how a māsaka would translate into current monetary rates. No one can answer this question with any certainty, for the oldest attempt to peg the māsaka to the gold standard dates from the V/Sub-commentary, which sets one māsaka as equal to 4 rice grains’ weight of gold. At this rate, the theft of an item worth 20 rice grains’ (1/24 troy ounce) weight of gold or more would be a pārājika offense.
One objection to this method of calculation is that some of the items mentioned in the Vinita-vatthu as grounds for a pārājika when stolen—e.g., a pillow, a bundle of laundry, a raft, a handful of rice during a famine—would seem to be worth much less than 1/24 troy ounce of gold. However, we must remember that many items regarded as commonplace now may have been viewed as expensive luxuries at the time.
In addition, there is one very good reason for adopting the standard set by the V/Sub-commentary: It sets a high value for the least article whose theft would result in a pārājika. Thus when a bhikkhu steals an item worth 1/24 troy ounce of gold or more, there can be no doubt that he has committed the full offense. When the item is of lesser value, there will be inescapable doubt—and when there is any doubt concerning a pārājika, the tradition of the Vinaya consistently gives the bhikkhu the benefit of the doubt: He is not expelled. A basic principle operating throughout the texts is that it is better to risk letting an offender go unpunished than to risk punishing an innocent bhikkhu.
There is a second advantage to the V/Sub-commentary’s method of calculation: its precision and clarity. Some people have recommended adopting the standard expressed in the rule itself—that if the theft would result in flogging, imprisonment, or banishment by the authorities in that time and at that place, then the theft would constitute a pārājika—but this standard creates more problems than it would solve. In most countries the sentence is largely at the discretion of the judge or magistrate, and the factor of value is only one among many taken into account when determining the penalty. This opens a whole Pandora’s box of issues, many of which have nothing to do with the bhikkhu or the object he has taken—the judge’s mood, his social philosophy, his religious background, and so forth—issues that the Buddha never allowed to enter into the consideration of how to determine the penalty for a theft.
"Thus the V/Sub-commentary’s method of calculation has the benefits that it is a quick and easy method for determining the boundaries between the different levels of offense in any modern currency; it involves no factors extraneous to the tradition of the Vinaya, and—as noted above—it draws the line at a value above which there can be no doubt that the penalty is a pārājika.
The Commentary, arguing from two cases in the Vinita-vatthu, states that if a bhikkhu steals several items on different occasions, the values of the different items are added together to determine the severity of the offense only if they were stolen as part of a single plan or intention. If they are stolen as a result of separate intentions, each act of stealing is treated as a separate offense whose severity depends on the value of the individual item(s) stolen in that act. This point is best explained with examples:
In one of the Vinita-vatthu cases, a bhikkhu steals ghee from a jar “little by little.” This, according to the Commentary, means that first he decides to steal a spoonful of ghee from a jar. After swallowing the spoonful, he decides to steal one more. After that he decides to steal another, and so on until he has finished the jar. Because each spoonful was stolen as a consequence of a separate plan or intention, he incurs several dukkaṭas, each for the theft of one spoonful of ghee.
If, however, he decides at one point to steal enough lumber to build himself a hut and then steals a plank from here and a rafter from there, taking lumber over many days at different places from various owners, he commits one offense in accordance with the total value of all the lumber stolen, inasmuch as he took all the pieces of wood as a consequence of one prior plan.