I'm not sure I can help here, but are you...
(a) working to judge distance by identifying a 100m length (range),
or
(b) working toward accurately pacing out a 100m distance?
Your note suggests the latter, but I'm not sure. If so, are you in an area with sidewalks? Many sidewalk squares in the US are 5' x 5', meaning there's about 66 sidewalk squares in 100m. Measure a typical to ensure its actual size. If you're not in area with regularly squared sidewalks, you might need the following method to get a 100m range quickly.
If you're looking to practice measuring "eyeball" ranges up to 100m, all you need to do is locate some objects with known heights off in the distance... these could be vehicles, doorways, or anything that's plus or minus a few inches of what you'd normally expect.
For example, at 100m, a 6-foot-tall person is about the height of your thumb nail, with your arm extended. A 2m tall doorway is about 1.5cm (.6") tall at 100m.
The formula for this is a little quirky, but relies on basic fractions. Take a ruler, hold it at arm's length, and measure the apparent height of the known object. The height of object X at a distance of Y is the same as the height of the object as you see it (a) over the length you see it (b). Or:
X a
- = -
Y b
X= height of object (2m)
Y= distance to object (unknown, in meters)
a = the height as it appears to you (1.5cm, or .015m)
b = the distance between your eye and the ruler... about .75m, but you should measure since arm lengths vary quite a bit
Multiply X times b (2 times .75 = 1.5)
Divide that by a (1.5 / .015 = 100)
Result? When a 2m doorway looks to be about 1.5cm tall, you're 100m away from it. Obviously, you don't need a doorway... take anything with a known height and substitute it for X. Just be sure to remeasure a.
I hope this helps.