Yeah, I looked at the Kawai regulation manual and that is partly what confuses me. Here's what I know:
1. The Boston spec for key height as listed in the "Steinway Worldwide Technical Reference Guide" is 65mm, but it doesn't say where to measure from and to. In the actual regulation sequence for Steinway pianos, it says to measure from the cutout in the key bed to the underside of the key cover. But it doesn't say specifically what to do with Boston.
2. Boston pianos are made in the Kawai factory, so it's possible that I should treat a Boston grand like a Kawai grand. The Kawai regulation manual says to measure from the key bed to the top of the key and their spec for Kawai grands is 66mm. (Remember, the Boston spec according to Steinway is 65mm, not 66...)
3. I called Kent Webb at Steinway and he said he thinks it is to the underside of the key cover.... He didn't seem to be sure...
4. The Boston Grand I am regulating is about 2 1/2 years old with heavy use in a practice room. The actual measurements at A0 and C88 were 64mm from keyed to underside of key cover and 65mm to the top of the key. I figured I would typically need to add rather than subtract, so I went ahead and made the key height 65mm to the underside of the key cover. I'm thinking that perhaps Kawai gave Steinway a spec number 1 millimeter less than their normal 66mm because they know Steinway measures to the underside of the key cover... But Steinway also measure form the cutout... Arghhh
I'm still in the process of regulating, so we shall soon see if where I set the key height will work in this piano.
In the meantime, I think I will email Kent Webb and see if he can track down the actual answer.