how to find my efficiency

on my last brew I was supposed to get a target original gravity post boil, of 1.075.
I actually got 1.086 so my 70% efficiency was set too low.
How do I find our what efficiency I actually achieved, so that I can hit the correct numbers next time

On your 'session' tab, you will find where BeerSmith has put the Mash and Brewhouse efficiency for the recipe along with the actual calculation based upon the numbers you have filled out for the brew session as 'meas' values.  I have included a screen shot with those areas circled. 

Looking at both is important, because you may find, as in this example, that the mash efficiency was right on but the brewhouse was not.  This is an indication of having the gravity numbers correct, but the volumes being off.  Likewise, if the brewhouse efficiency is close, but the mash efficiency is different, it may well be an indication that you need to rebalance your volumes to account for low/high mash efficiency numbers.

thanks for the reply, while looking at your image, I noticed something on my mash page, which I don't know what to do with, or where it comes from.
That is the "measured pre-boil volume"  am I supposed to know this, or is it worked out for me?
My brews are all 50 litres so it looks half what it should

Greetings moog - whenever BS uses the word "measured" it's asking you for input.  Once you're finished sparging, enter the Pre-Boil value into that space.  Above Measured Pre-Boil Volume, is Estimated Pre-Boil Volume.  That's the BS estimate for that value.

In order to get BeerSmith to properly calculate your actual efficiency you will need to measure and enter this volume.  To measure the volume, some people use a calibrated dip stick (like me) with the volume of the pot marked out.  Others measure and mark their pot or kettle or purchase a kettle with the volume already etched or embossed into the side.  Either way, the more accurate your measurement can be the more accurate the calculation of mash and total efficiency based off it will be.

ok, thanks for the help guys,
So, with the values of measured pre-boil gravity, and measured pre-boil volume,  I use these measurement figures to improve my brew figures.

Up until now, I've been mainly checking my gravity at the end of boil, then adjusting up my total efficiency for next time.

I read recently on a forum that the best time to take a refractometer reading for pre-boil gravity, is 10 minutes into the boil, after its fully mixed and cleared a bit, do you have any advice on this?
By the way, I'm doing BIAB.

I do mainly BIAB as well.  I take my pre-boil gravity after pulling and draining the bag as the pot is coming up to a boil.  Once you start the boil, the gravity reading is dependent not only on your mash efficiency but also on your boil off rate.  As the wort is coming up to a boil, I can stir the wort and take a sample for a hydrometer reading.  As that is cooling, I also take refractometer readings as well, usually 3 to 4 to make sure they are consistent.  I use this refractometer reading to help adjust the boil time as needed if I am off just a bit.  It takes me about 20 minutes to cool the wort sample down for a hydrometer reading and about 45 minutes to bring the pot up to a boil.  With lots of brews under my belt, I have the timing down pretty good.  My whole process is as follows:

Remove bag from pot and hang to drip for 5 minutes during which I check conversion with starch test, heat goes on under pot.
Squeeze bag for remainder of free wort (another 5 minutes)
Use dip stick to read and record volume of wort (2 minutes)
mix and remove 300 ml sample in canning jar for hydrometer test and place jar in bowl with running cold water (3 minutes)
stir again and take 3 to 5 refractometer readings (3 minutes)
Shut off running water cooling down sample jar and fill bowl with ice and water (2 minutes)
Weigh out hops for each addition (10 to 15 minutes)
Check temperature of wort sample (usually it is within a couple of degrees of target) (2 minutes)
Take hydrometer reading, put pH probe in excess wort for pH reading (2 minutes)
return samples to pot, update information collected in BeerSmith from brew day sheet.
At this point the pot is close to or just starting to boil.