You are doing well.
My 93 300D averages 34 mpg. I have the 18.5 USG tank in mine. Taking it down to .5 gallons left would give me 612 miles per tank. I usually chicken out though and fill up around 500-550 miles.
I haven't taken it on a trip yet, maybe my MPG would be better.
Do you use any additives? I'm using 12oz DK w/ Cetane booster per tank full.