I'm curious as to what sort of yield you get from pressing whole clusters in a basket press? I recently pressed out my first batch of white juice using crushed/destemmed grapes and got around 100 US gal/ton (~415L per metric tonne).
I crush my grapes. Not whole cluster pressing.
But I do track volumes so I can answer this question. Sort of. Because different grapes have tighter clusters than others so the first volume measurement** --- harvested grapes in a bin --- is rather variable. That being said, basic rough numbers:
Crushing of my grapes into must reduces the volume of harvested grapes to about 45% of the un-crushed grapes.
Wine after pressing is about 28% of the un-crushed grape volume, and about 60% of the must volume. Which makes sense as most of the crushed grapes are liquid not solid, so most should come out into liquid.
This is for white wine not de-stemmed. De-stemmed grapes will reduce the must volume after crushing quite a lot, but should give about the same total volume of juice after crushing. And if one ferments on the skins, one will get even more wine as the skins break down and give up more juice.
So if one takes 1 metric ton is 1000 KG and one assumes the wine is 1 KG per liter, than doing the math.... assuming an equal volume to weight ratio (which is only an assumption*), and using actual liquid volumes and weights are "given" than 28% of 60% of the original volume is about 46%.. Your results were about 41%. Which is just fine.
And if you look online, you will see that most pressed wine weights come in at about 35%-45% of the grape weight. So you are fine.
Hope this helps.
* Often in science one may have to consider, for purposes of the thought experiment, an Elephant of negligible mass....
** I deal with metric tons of grapes, self harvested, But I do not weigh them ... which would be better, but I do not have a 1000 KG scale....
But I have taken samples of grapes 20 L and weighed them, and I have a sort of base line, and the results are similar... as in close enough in the horse shoes and hand grenades scale. Which is fine for me.