We also use the variable cost but it does come up rarely. Moreso if you use talents to get out of norm spell lists.
Also on page 64 there is the cross marker on the spell lists noting that the DP cost might change so while the page is an easy quick reference, it isn't correct.
I never noticed the JD's spreadsheet didn't take it into account. I have had it go into effect with one character, a fighter who got lists via a talent, and tracked it manually. He is an older character and we were using a differet spreadsheet that is a bit more complicated but does have some advantages.