the footage given on the putter is a guide based on a flat surface and a standard green speed. therefore any other green condition needs to be taken into account. uphill, downhill, left break, right break . also slow, fast and very fast green speed. all this effects the distance.
now i'm well aware you know this but you should (IMO) treat the green speeds as follows:
- Very fast = fast (less than putter footage)
- fast = standard (same as putter footage)
- standard = slow (more than putter footage
- slow = very slow (much more than putter footage)
hope this helps.
oh and also i find with putting on "very fast" that the longer the putt the much less you need to hit it. so if you have a 15ft putt hit it about 12ft, 30ft putt hit it about 26ft. then if you have a 60ft putt only hit it 50ft.