As always, the devil is in the details, and this is one of the most highly detailed piano jobs. I find the Mother Goose level (adapted from the original Fazioli tool - they have modified theirs, but it is no available) not very user friendly. It is necessary to place the level as close to the strike point as possible, which means right in front of the dampers. That is usually troublesome, because you have to lean over to see the bubble, and reaching in to pick it up so you can hook the string(s) is relatively inconvenient. It also won't give you a reading on the unisons next to the strut, which are commonly the worst offenders.
So I made my own level, after a somewhat similar design of Terry Otake (Shigeru MPA). It places the bubble where it can be easily seen, and it sits right next to the strut to catch those strings. It does need to be very lightly supported while plucking the string - finger and thumb just touching, next to no pressure. This is actually very ergonomic, as the finger and thumb are in place to pick up the level while hooking and pulling up the wire.
I've done this on at least dozens if not quite hundreds of pianos over the past 15 years, always successfully except in a couple instances where strings had been grossly over-pulled by someone previous. I have found that it always makes the mating process go more quickly, and that in less than the most picky situations I can get by without bothering to mate, as long as I have checked that my crowns are quite level. On good days, it takes under 30 minutes to do a meticulous job.
I find it faster than leveling the strings to the hammer, as it is a more direct measurement: you don't have to raise the hammer, and worry about just how much you are pressing the hammer toward the string. But I realize that different folks are more comfortable with one thing than another, and there are lots of ways to go about this particular job.
I've also fooled with under string levels, so I'll include photos of three I designed and made. They all work pretty well. I find I just revert to my bubble level, though.
Regards,
Fred Sturm
"The true sign of intelligence is not knowledge, but imagination." - Einstein