Putting distances seems to be pretty formulaic. Estimating break is way more about feel. I don't make a whole lot of long putts, but I have gotten pretty good at leaving them inside of 2 feet using this formula:
Effective distance is the length of putt in feet plus the elevation change in inches. So, as the above example, a 24 foot putt with 4 inches of downslope is effectively 20 feet. That is the starting point. Now adjust for green speed as follows (EDIT: this is based on the same ratios used by TigerPaw above, slightly fudged for ease of calculation)
9 subtract 10% (tenth)
10 subtract 20% (fifth)
11 subtract 25% (quarter)
12 subtract 25% if uphill. Subtract a full third if downhill
13 subtract a third (i fudge a little extra on downhill)
14 pray
If the putt is severely downhill, drop another 5-10%.
High wind on longer putts can be worth a few extra feet.
E.G. 49' putt on St. A. Hole is 13" below me. Green Speed 12. Wind in my face 25 mph.
Effective length is 36'. Drop a third (12') for a speed adjusted distance of 24'. Add a foot for wind. Pull up the 100' putter scale and go to the quarter mark. Should be pretty close.
If the putt was 13" uphill instead:
Effective distance is 62'. Only drop a quarter (~15'), add one for wind and hit it 48'.
This is also a good example of why you have to be more conservative on downhill putts. From the same distance to the hole, the downhill is at least twice as sensitive to error.
CAVEAT! The elevation changes are given in inches up to two feet. At that point, 24"-30" (I think) reads 2'. 31"-42" reads 3' etc. Be careful. Also, the difference between 0.1 inches and 0.9 inches, coupled with the built in variability in putters and balls means that a "flat" putt of 10' on a Speed 11 green hit exactly half way on the 15' putter scale might be anywhere from 2' short to a little long.
Remember, this is for lagging. Trying to make a 12' putt involves being more aggressive with the distance. Add several more feet to make sure you don't fall prey to the estimation gaps and equipment variability. (And it cuts down break, but that is a different issue) Nothing is worse than a 10 footer that dies at the lip.