For lag putting I use my right foot, since I'm right-handed, as distance gauge. You'll need to practice this, but what you do is determine how many inches off your right foot you need to bring the putter back to hit the putt. For instance if you have a 40 foot putt, then I bring the putter head about 4-5 inches off of my right foot's baby toe. If it is a 30 foot putt, I'll bring the putter head about 2-3 inches off my right foot's baby toe. For a twenty foot putt, I'll bring the putter off my right foots baby toe, for 10 foot maybe it's the middle of my right foot where I bring the club back to. Again this guage works for me. The inches off your foot is an approximate and you'll need to determine this during your own practice sessions.
At the practice green I'll work on this lagging drill from different known distances, until I can kind of determine how far to bring the club back for each know distance of a putt.
Now when I'm on the course, what I will do is pace off my putts looking for anomalies in the putting surface as well as determine how far of a putt I have. Know that each pace is a yard or 3 feet. If I count off 10 paces that's 10 yards or 30 feet. I'll add or minus some feet depending on the slope of the putting surface. By pacing off the distance of my putt, allows my brain to register how far of putt I have and solidfies exactly how far I have to roll this.
Then once I know how far the putt from the hole, my brain knows what a 40 ft or 30 foot putt feels like. If I'm ever unsure I've got my "feet" gauge to rely on from my practice. This has really improved my putting and helped me get from a 21 handicap to a 10 handicap in about a year's time.