@SpiderCrowWolf, it's not actually math in this case; the 125sp is spent by buying the Not So Routine Machine on the DD first. When the set comes up after that, you should get it free if you only have that item.
Since you already have items from that set, it may or may not be cost effective to buy the PC on sale -- I recommend using the calculator in the other thread to work out what your price would be when the DD comes up without buying more items.
Here's the bit in the original post about the PC item (my emphasis):
"LadybugLeslie;809797" wrote:
Mid-Century Modern Trailblazer- 125 total SP
- DD #460: Not So Routine Machine (125 SP)
- DD #551: Mid-Century Modern Trailblazer -- Free-- 1250 SP set for 125 SP
