Certainly the palatability of a standard Propylene Glycol product can be a challenge. It is very bitter and as a cow has two and a half times the number of taste receptors than a human, she is very quick to pick this up. She can distinguish four primary tastes salty, sweet, bitter and sour. So it can at too high a level actually supress intakes. However fed correctly, a blended, specifically formulated product can remove any intake suppression.
Feed rates depend on what you are trying to correct? Studies show propylene glycol increases milk production, and reduces the mobilisation of adipose tissue, but there is very little conclusive evidence that it actually decreases the incidences of Ketosis. Academic studies/papers would suggest feed rates ranging from 200-700g/head/day, (even 900g/head/day in early lactation). The concern with feeding at these higher levels, is the lack of research, data available with regards to the impact on overall dry matter intakes.
On the basis that the base diet is balanced both chemically and physically, intakes are good, and visits are at the level we would expect then we would look at 250g/head/day (0-30 DiM) and then 100g/head/day (31-40 DiM).