I added two 10% increments (6.5+6.5)... but instead of adding 0.65 (1%) seven more times, I added a 5% increment (6.5/2 = 3.25) and then 2 increments of 1%
So 6.5+6.5+3.25+0.65+0.65 = 17.55
I still had to use a calculator to add those weird numbers (and also check my work), but it does seem really practical for easier numbers. I usually need percentages for pricing (i.e. discounts/tipping), and the percentages are normally in increments of 5%, so that's pretty useful for figuring out a 15% or 75% of something real quick... or at least get me really close (when talking about something like $X.99)
Regardless, I appreciate the head trick!
Edit: I guess I could've done 30% and then subtracted 1% twice; but it's the same issue (of adding weird numbers) with the same outcome anyway. So thanks again!