I had read that hop oil isomerization is optimum in a wort of approximately 1.040. Don't know if the information is still valid or just something that gets repeated. I'll have to start looking around for verified information with a more current date.
I've been starting with one pound of LME/DME per gallon for the 60 minute boil. Seem to produce a not too dark beer. The remainder goes into the boil kettle at 15 to 20 minutes left.