What is the ideal way to update an old value to today's currency value? For example, how much is $25 million in 1790 considered today?  How do I update such value?  It doesn't make sense to me to use, say, price indices.  Shouldn't such update be done based on the expansion (or contraction) of monetary supply?
Clarification post-comments.  My thinking --- please educate me if it is nonsensical --- is that inflation is an expansion of the monetary supply, so I would like to do a computation from the phenomenon of inflation itself, not of its effect, one of which is an increase in prices. So I would like to be able to estimate a loss of value merely from the the increase in monetary supply.  I could then compare it against real facts --- which would be the market prices.  I'd be willing to read an entire book just to learn how to do this.  So feel free to advise.  Thank you!
I think this question stems from misunderstanding from what inflation is. My best guess is that this happened because most macro books will say that inflation is a monetary phenomenon as in mainstream economics it is accepted that inflation over long periods of time is affected mainly by money supply. This is illustrated by equation of exchange (see Blanchard et al Macroeconomics an European Perspective) which is given by:
$$P= MV/Y$$
This is the same equation that Brian also mentioned in his comment just solved for prices. Here $P$ are the average prices, $M$ money supply, $V$ velocity, and $Y$ real output. The standard textbook argument for inflation being monetary phenomenon goes like this. Velocity is assumed to change over business cycle but not really over long run and hence it cannot be responsible for all inflation and we can hold it constant. Moreover, real output tends to increase over time which should be deflationary. Hence the argument goes that over long run inflation should be caused by $M$.
However, this does not mean that inflation is defined in terms of $M$. The definition of inflation is simply the change in average prices for example:
$$\pi_t= \frac{P_t-P_{t-1}}{P_{t-1}}$$
This is what inflation is. Moreover, inflation also is measure of how money looses value. The issue is that money especially in short run might not loose value just because there is more of it (i.e. by $M$ increasing) but also because velocity changing or real output dropping. Furthermore, $M$ and $V$ are not always independent. At zero lower bound any increase in $M$ will be offset by changes in $V$ because at zero lower bound peoples preference for holding (but not necessary spending) cash will be infinite (abstracting from issues such as storage costs for large sums of cash). What even more, models that are above simple undergraduate textbooks rely not just on actual values of $M$, $V$ and $Y$ but also on peoples expectations of these (see Krugman 1998).
Hence you simply cannot calculate how money looses its value from $M$. At minimum to calculate change in $P$ using $M$  you would need to know and control for not just what $M$ is but also what real output is and velocity is but here you will run into problem that real output is usually calculated by deflating nominal output by CPI - so you introduce the same problem you wanted to avoid. What even worse velocity itself is usually not directly observed but imputed from data. And this is all before we start factoring in all the expectations we should that are extremely difficult to measure. Compared to such convoluted way using CPI directly is far more sensible even if it may be imperfect measure.