Volume displacement equivalents of DME/LME

Is there a simple calculation or "rule of thumb" to help figure out how much volume a given amount of DME or LME will take up in a kettle?

For instance, (arbitrarily) I have a recipe that requires 3lbs DME and 4lbs LME. I want to do a full-boil with beginning volume of 6 gallons which will reduce to 5gallons with 1gal/hr boil-off.

What I dont know is how much water to boil prior to adding the DME and LME? I assume I would need to have less than 6 gallons of water to compensate for the volume of the DME and LME.

I think I read somewhere here that "roughly" 3lbs DME and 4lbs LME is the rough equivalent of 1 quart of water.

If this is true, then I would need to have 5.5 gallons water and the remainder of the volume to 6 gallons will be extract?

Any ideas on whether this equation is correct/close?

DME will increase volume by a few cups. the LME will increase the volume by the volume of the LME (If you have a 1 gallon jug of LME, it will increase the volume by 1 gallon).

a quart of water is not 1/2 gallon, it is 1/4 gallon.

honestly, just boil 6 gallons minus the LME volume, make your additions, and then start your 1 hour timer when it boils back down to 6 gallons.... extra boiling is not a problem, and I don;t think that the extra volume of the DME will even be noticeable by the time it gets up to a boil.

This is one of those aspects of brewing where the idea "Relax, don't worry..." comes into play.

Some things are worth splitting hairs over. This is not one of them.