6oz of DME really is an O.G. of 1030... I've always done that but, never actually measured..
I do all of my starter wort in quart canning jars. 2.5 oz per jar. 15 min at 15 psi. I do about 14 quarts at a time. Always room temp when I need it.
even better, next time you are making beer calculate for an extra 2 gallons of wort. run off into a separate container and dilute to 1.030ish and pressure can. I find I can often just run the extra water through my grain after I get my boil volume and get what I need. free, sterile, room temp starter wort!