Everyone has their own way of valuing miles. I'm sure that some here would see a $25K fare for JFK-NRT and redeem 80,000 miles just to say they got 31 cents/mile of value out of an award. Ignoring the fact that they never really would have spend $25K on the ticket and wasted 80K miles.

I have a lower threshold - I've been earning miles faster than I can use them for over a decade now - 1M+ in Marriott, 600K at Starwood, so I use them for almost any vacation the family takes (no, not stupid things like $99 cat 5 rooms), so I tend to redeem at a lower value (10K/night vs. $250 is typical).

Every situation is unique - your balance, earning rate, outlook on future valuation, etc., all factor in.

On your basic question, "how to calculate that value" - for the above scenario, I'm spending 10,000 points instead of spending $280 or so (with taxes, fees, etc.) so the points are worth (280.00/10000=) 2.8 cents each