Last runnings help

2nd batch using Beersmith.  I undershot my gravity on the 1st batch due to misunderstanding how beersmith calculates efficiency versus Promash.  But made the correction for this batch to 77.8% and hit my volume and gravity on the nose (13.47gallons at 1.034).  But, this time I decided to take a last runnings gravity and it was 1.004!  Anyone know a quick way to calculate how much to lower my efficiency rating to still be able to hit my volume and pre-boil gravity, while raising my last runnings a bit?  I would rather not use trial and error if I dont have to.  Also, I am fly sparging and dont plan on batch sparging.  Thanks for any input.

I have never use Beer Smith to calculate my mash efficiency. I usually figure the temp and Ppg of my mash divided by maximum Ppg of the malt used. (John Palmer's How to brew page 192) Don't know what temp you tested the gravity of your last running but 1.004 is close to straight water. I  would top off with distilled water after wort is boiled or just make a smaller batch, depending on the gravity after Boiling, and not use your last running. Remember a hydrometers specific gravity is calibrated at I believe 65 to 70 degrees.

In hindsight I would have definitely cut the runoff and diluted in the kettle. I am in the market for a refractometer now to make reading the last runnings easier (dont have an easy way to pull from my grant). I think part of the low running number was due to this being my first low gravity beer.

My mash effeciency has always been around 84%. Which in beersmith lead me to a brew house effeciency of 77.8%. However, I want to lower that to around 71% to keep me from running into this issue again. I am thinking about opening up my mill gap (currently at 0.045") and adding a couple lbs of malt to get both my volume and gravity, as well as a lower effeciency. I assume if I left my mill alone and just add malt to raise my last runnings I would have to constantly monitor my volume and gravity looking for the perfect point to cut the runoff and add water to avoid overshooting my gravity?  Any thoughts on opening the mill gap to lower effeciency?

I've never used promash or messed with efficiency so I'm just thinking out loud here but if you have a recipe that you used through promash, couldn't you put that into beersmith and tune the efficiency to hit what you know you got before?