I guess this is where I post something about me.  I have no mathematical formulas - it's sheer luck and attachment to sentimental favorite numbers.  They come around about 4 or 5 times a year and I got hooked when I used to hit on them almost on schedule to the day!

I usually include a set of five st/bx QPs in the mix, and it has hit every now and then when I least expect it.  I only recently started focusing more on doubles - for the half price Wheels, and triples when my spidey sense is tingling.