Hitting the numbers

I brewed a batch of Irish Red today using a recipe I developed using Beersmith.  I am trying to achieve better efficiencies and get the volumes dialed in so I can more consistently hit the predicted numbers.

On today's batch I hit a pre-boil gravity of 1.039 against the predicted 1.037 so things were heading in the right direction.  The wort quantity was spot on into the kettle too.  I did a 60 minute boil and the boil-off was as predicted (<0.1 gallon error) but the post-boil gravity reading which should have been 1.044 was 1.041.  I took the reading several times - in the kettle and in the fermenter.  The measurement temperature for both, and the refractometer the same, in both measurements. 

What would cause this error and how can I hit the numbers predicted by BeerSmith more consistently?

Paul

One thought is your pre-boil measurement could be off.  We did a brew 2 days ago (15 gal batch) and the pre-boil gravity taken from the bottom of the kettle was .020 higher than that taken from the top.  Once we stirred to mix, it came out near the middle. 

If your post boil volume is off your post boil gravity will be off as well.

Are you using a pre-printed volume markings on the side of your kettle for the volume measurement?  If so, I would recommend checking them before you rely on them too much.

Secondly, make sure your refractometer is clean.  Are you reading directly as SG or are you reading Brix and converting?  If you are reading the SG directly from the refractometer, I would recommend reading the Brix number and using the refractometer too in BeerSmithl to convert to SG.  Especially with the cheaper refractometers, the scale for SG is not usually that accurate.  I chased this a bit with my first refractometer before using the BeerSmith tool and my hydrometer to re-calibrate the refractometer to match my hydrometer.  Now I just record the Brix reading and convert when I get the time.