Many bloggers propose using a rule of thumb based on a "value" threshold that is just very flawed in its construction. Don't follow such a questionable rule IMO.
You can make it as complicated as you like. Most people just want a rule of thumb to avoid acting and feeling stupid when spending their points.
And FWIW, if you spend too much time calculating the points to be earned between base points, status bonuses, promos, credit card, etc. + elite status considerations... you soon find yourself with millions of points and no "breakeven or better" chances to spend them.