Assuming the Lexi Challenge is 13 speed greens you have to use the right multiplier for that speed (I use .7)
For uphill putts you add the distance + rise in inches + a little extra, then multiply that by .7 for 13 speed greens. (exp: 7 ft 3" up + 1ft = (7+3+1)*.7=7.7 foot putt on your scale.
For downhill you take the putt distance - slope in inches, sometimes you need to subtract additional feet. (exp: 8 foot putt 2" down = (8-2)*.7= 4.2 foot putt on your scale.
Here are some of the multipliers for different green speeds that I use:
7.0 (speed) ~ 1.25 (multiplier)
8.0 ~1.1
9.0 ~ 0.95
10.0 ~ 0.9
11.0 ~ 0.81
12.0 ~ 0.78
13 ~ 0.7
Hope this helps and doesn't confuse you.