The essential problem in translating currency across large spans of time is that the relative values of different goods and services have shifted drastically in the past few centuries. Modern improvements in productivity have drastically reduced the cost of many items, but the shifts have been of very different magnitude. In addition, the growth in wages has led to the cost of labor-intensive services increasing by a great deal relative to the cost of durable goods (this is known as Baumol's cost disease).

To give an example of this, let's say I have $20 in present-day US currency. With that, I could buy a wool sweater, or a hatchet, or transport from Boston to NYC, or half an hour of a tradesperson's labor (perhaps less). If we compare that to prices in England circa 1300, the wool sweater would cost a few shillings, and the hatchet would cost 5 pence. Transport costs are a little tougher, but (for a trip of a similar distance) the cost for a hired cart alone might have exceeded half a pound, even before costs of food and lodging along the way. But a half-hour of a skilled tradesman's labor would cost less than a quarter-penny.

So as we can see, four things that have similar cash value in the modern day could differ in value by a factor of up to 1000 in the medieval era. Which is why estimates such as the ones you quoted can vary so significantly. My advice, to answer the actual question, would be to index by something that makes sense for your purposes, and use it consistently. The price of land might be a good one, as it relates directly to the tax. Perhaps you could look up the current tax assessments for the area and use that as a comparison? Or since this is for the general public, using wages might give them a better sense of how long an average person back then would have to work to earn that sum of money.

Realistically, comparing currency values in this way is always going to have a certain amount of fiction to it, so there's never going to be a "correct" answer. So whatever gets the point across to your audience is the best method.

Sources for anyone interested:

Hodges, Kenneth. "List of price of medieval items." http://medieval.ucdavis.edu/120D/Money.html

Masschaele, James. “Transport Costs in Medieval England.” www.jstor.org/stable/2598017.

You really should read the essay on the website that describes not only what these differences mean, but the kinds of things you need to keep in mind to make sense of these numbers. It's well-written and well-explained. To make any real sense of historical currency value, especially for something so many centuries old, you really need to understand what you are doing — it's not a simple thing of plugging in a number and getting another out, because value is a changing thing over time.