Source: an anonymous manuscript preserved in the Padua conservatory, thought to be mid-18th century. (Reproduced here below)
“To tune the harpsichord
C-G; G-D; D-A; A-E: Fifths. It is appropriate that they are narrower than the exact fifth, which means that the higher string has not to be pulled up to the right consonance but instead is left quite loose and flat.
C-F; F-Bb: Wider fourths
Bb-Eb; Eb-Ab: Narrower fourths
E-B; B-F#; F#-C#: Perfect fifths
C#-G#: Test. so it makes a perfect fifth
In this partition, major thirds are a little altered in the plus and minor thirds in the minus”
[Translation: Fabrizio Acanfora]
As with almost all mollified meantones, the instructions are somewhat open to interpretation, but the logical limits leave very little room to move. If the first fifths are not significantly tempered, there is no requirement for the two larger-than-pure fifths. One extreme is to make the first major third C-E pure (upper graph). The other extreme can be constructed by assuming that an equal-tempered fifth is about the least-tempered that can be easily heard as being tempered, at least in a historical context; therefore, the two wide fifths can be taken as being ≈ 2 cents too wide. Assuming all the narrow fifths are of the same size, they would have to be tempered by 1/6th Pythagorean comma (lower graph). I don’t mean to imply that this is a rational circulating temperament, only that a mathematical approach returns these results.