As it happens, the ratio by volume method of preparing must for mead making is very close to my heart and I use it all the time. This method of describing mead sweetness is still used in Poland and it goes to the old days of mead making in that part of the world. I think it is more reliable and accurate than so called egg method used in other parts of Europe and allowed to rough estimate of sugar content in the must preparation. Back then hydrometers where none existent or unknown to mead makers and they had to rely on some reliable and repeatable method of calculating how much honey and how much water to use in mead making. To this day in Poland mentioning the ratio of honey to water in any mead related conversation is immediately understood without ever mentioning Specific Gravity. Also, mead is officially (legally) named and divided according to the honey to water ratio and the names like Czwórniak, Trójniak, Dwójniak or Półtorak describe the level of sweetness. So, instead of saying dry mead, semi sweet mead, sweet mead or desert mead - we would use the name of the ratio - czwórniak, trójniak, dwójniak and półtorak accordingly. There are other ratios on the dry side of the spectrum but in Poland these types of mead are almost never made. I should also mention, that while you count the "parts" in the ratio, you count the honey part and the water part and it is the total count of the parts which creates the name.
Lets take a look at the ratios, their names and relative Specific Gravity:
Czwórniak - means four parts - 1 part of honey and three parts of water - SG range 1.1057 - 1.1244
Trójniak - means three parts - 1 part of honey and two parts of water - SG range 1.1340 - 1.1637
Dwójniak - means two parts - 1 Part of honey and one part of water - SG range 1.2055 - 1.2500
Półtorak - means one and a half parts - 1 part of honey and half part of water - SG range 1.2734 - 1.3291
As you can see, there is no SG lower than 1.1057 mentioned above. There is no mistake, now days there is no company that I know of which wold produce mead commercially at this level of sweetness. Historically, such a mead would be named Piątak.
Piątak - means five parts - 1 part of honey and four parts of water - SG range 1.0654 - 1.1011