To put on each 10kg of 'stores' at 20% water, you'd have 8kg of sugar in that 10kg of stores.
With 2 pounds to a pint (62%?), that would be 8 x 100 / 62 = 12.90 kg of imperial 2:1
However with an invert syrup at about 73% sugars, to get 8kg of sugars, you'd need 8 x 100 / 73 = 10.96 kg of syrup.
So to get the same amount of sugar into the hive, you'd feed 10.96 x100 / 12.90 = 84.96% (call it 85%) of the syrup weight you'd give with "2:1" white sugar - ie about 15% less weight of syrup.
With invert, you need a bit less syrup weight, about 15% less (who said "less than 20% less"?).
Which is about 15.3kg instead of 18kg ...
However, it'd be a mistake to think that you can calculate exactly how much syrup to add, as the bees are still foraging and still consuming, so, without feeding, their daily hive weight will still be changing quite a bit from day to day.