Today as this was a my first go at the full capacity of what this system can do I dialled the BH efficiency back down to 75

Today's example: expect - 13 gallons at 1.043 into boil kettle, after an hour boil and a 1 gallon boil off expected OG 1.048

Actual: 13 gallons at 1.047 into boil kettle, actual boil off approx 1.125 gallons and OG of 1.049

Just trying to follow your numbers. You can do a simple gravity point balance to check if there is something wrong with your calculations.

You measured 13 gallons at 1.047 gravity pre-boil.

Post boil, you ended up with 11.875 gallons at 1049.

Cooled down (I am assuming that you took the measurement at near boiling temperature and your thermal expansion in BeerSmith is set to 4%) you would have 11.42 gallons.

So pre-boil, you had 13 gallons at 47 points = 611 gravity points.

After the boil, you had 11.42 gallons at 49 points = 560 gravity points.

The 51 point difference is about an 8% error.

These numbers should be the same or near enough with measurement error. I am guessing that your post boil gravity is pretty accurate and that your errors are either in your volume measurements or your pre-boil gravity measurements. The calculation above also is based upon how BeerSmith does the calculation of the pre-boil gravity from the batch size and the loss to trub and chiller volumes.

When you are trying to nail down your equipment profile to give you the most accurate prediction of your system, it is important to note the critical calculations in BeerSmith are based upon the volume to the fermenter and the Brew House Efficiency (BHE). These figures determine how much of the sugars from the mash end up in the fermenter. The program then calculates the subsequent losses in both volume and sugar content to back calculate how much sugar needs to be extracted from the mash to satisfy this starting goal. It will then calculate out the mash efficiency, pre-boil gravity and volume from that perspective.

This is why, as BOB357 and Kevin58 both stated, you will need to make sure your basic equipment profile is correct in the BHE and measurable volume losses. To do this, you should make sure that your measurements of gravity and volume are accurate as you can make them. To this, I recommend double checking any volume markings on your vessels if they come pre-etched or stamped. Regardless of the manufacturer, these are all pretty much produced from an engineering drawing and will have some variability in manufacture. If you want a method for doing this, I outlined my method here:

http://www.beersmith.com/forum/index.php/topic,20873.0.html on the BeerSmith forum.

Knowing accurate numbers for your boil off rate, grain absorption rate, and post boil losses are critical to making the program predict your results more accurately.

Another thing that you may want to consider is that there is a known error in the way BeerSmith calculates the pre-boil gravity. To do this correctly, the hot post boil volume must be used at the post boil gravity in order to compensate for the thermal expansion of the wort. The program, as noted above, takes the cold volume post boil and uses that to do the calculation of gravity. By doing this, it will always give you a gravity target which is off by the factor of the thermal expansion. For most people this error is not significant but when you are trying to dial in your system, it will take longer to arrive at consistent results.

If you need any help with the calculations or can't make sense of the numbers, feel free to repost here and we can try to help you out.