The typical PP fan understands the PP in terms of economic cycles, because that is how Harry Browne laid it out.
I have a different theory of its working, which I think of as "raid and stockpile."
A portfolio on this premise will contain any volatile assets and any "safe storage" assets in some percentages, and then have rebalancing bands. The net effect of the rebalancing bands is to buy the dip on the risk assets when they are down, and to shave off winnings to buy more safe stores of value when things are doing well.
The HBPP could rightly be conceived of as, stock as the risk asset, and then bonds/gold/cash as the "safe storage" assets. It is a conservative portfolio because it is 25% risk asset vs 75% safe storage.
However, the entire point of my post here is to point out that the HBPP is just one example of this type of system.
So now as to your concern - you're not sure how silver fares, and you're trying to orient yourself to a plausible role for silver in a modified PP.
Based on SLV data from 2006-2020,
I see low CAGR, effectively a price oscillating around a value.
Further, I know that silver has been seen as an exchange medium and store of value since time immemorial.
https://en.m.wikipedia.org/wiki/Silver#History
Lastly, I know that silver is a commodity you can physically own in your own hands, removing counterparty risk.
With that in mind - I do not know precisely how it will fare, but I can say with pretty high certainty that it is a stable store of value and not a growth asset, and that if you shave off stock market winnings and put it into silver, you are unlikely to regret it during a bear market or, SHTF situation.
How much silver? Dude I don't honestly know, but it will still not be a terrible idea. To determine that I'd first consider what percentage of growth-vs-storage you are looking for, then maybe look at equal ratios of all of your safety assets as a starting point, then adjusting from there.
Will it increase CAGR? That is what I assume you are backtesting to try to figure out, and IMO the answer is, "who cares?" Reason being, I would not look for CAGR improvements in the cold storage portion of your portfolio, it does not make sense. I would look for staying power and low correlation with the stock market instead.
I can't say these are HB's ideas, but they have helped me understand and orient myself when performing this kind of analysis on how to incorporate a new asset class into my investing.