DME typically has a "pound per gallon" potential of 1,046. That means that if you dissolve a pound of DME in a total gallon of syrup you have a syrup with a density of 1,046 (where 1 is water).
In metrical terms, the potential can be expressed as "kg for 10 litres" and is 1,038, which means that a kg of DME dissolved in a total 10 liter syrup has a density of 1,038 (or 1,0367 depending on whom you ask to, or what DME is on the table).
The easiest way to make calculations is to reason in terms of "total density points". If you could dissolve 1 kg of DME in 1 liter (I don't know whether you really can, probably not, but that's not the point) you would have 367 density points (1 litre with 1,367 density).
You can consider those "367 points" as your "solid total matter" in your syrup.
If your syrup is 30 liters, your density with that quantity of DME has a density of 367/20 = 18,35 points (1,01835).
If your syrup is 22,56 litres, its density will be 367/22,56 = 16,26 (1,01626).
2,7 kg DME in 23,67 total litres syrup will have a density of 367*2,7 / 23,67 = 41,86 points (or 1,04186).
If you want to raise your density by 6 basis points and you have 23 litres you need 23*6 total points = 138 points.
Considering that 1 kg will add 367 total points to your wort, in order to add 6 basis points to 23 litres you need to add 138/367 or 0,376 kg of DME to your beer.
(very strictly speaking the DME that you add will add a bit of volume to your wort so your density will be a bit less but when you calculate DME additions that's negligible).