OtiFernandez: Slow= -10%
What do you consider a slow green?
These are greens indicated as "Slow" in the top right corner.
OtiFernandez: And what does that -10% means?
This means to reduce the putting power by 10%. But, it's bad advice IMHO, as most of these tips are bad advice.
The only proper advice is for Standard greens. There, the putting power will translate to the same putting distance 1 / 1. E.g., if you pull the meter to 20% on a 50 ft scale, the putt will go 20% of 50 ft: 10 ft. Thus, in order to hit a ball into the hole at 8 ft, you hit it for two more = 10 ft which is 20% on a 50 ft scale. The two extra feet are to savely reach the full distance.
The other green speeds differ by 10 % from each other, in the top by 5%.
"Slow" needs 10% MORE power, so you'd have to another .8 ft power to that 8 ft putt: 10.8 ft.
On faster greens, you'd have to deduct power, but rather in % of the distance than by a certain value. If you deduct 7 ft on a fast green from the 10 ft power necessary for the 8 ft putt, the balls would stop after 3.3 ft ((10-7)+10%), and so forth.
And then there are the elevation slopes - another story...
There are a whole bunch of better putting tips - start with WGT's elaboration on the right =>