Beersmith question

Ok so today I brewed a wheat recipe that I cooked up for my first ever crack at a personal recipe. I am a partial masher with a 2 gallon coleman cooler mash tun. (works well for about 3 to 4 pounds of grain and a partial boil) I have my equipment set up at with 2 gallons MLT with 0.07 gallons (measured) dead space. My kettle 5 gallon aluminum brew pot with 10% evap rate and .26 gallons lost to trub. I brewed a batch prior to this one and hit the O.G. pretty spot on but this wheat beer I missed the estimated 1.044 by 1.039 O.G. I measured the gravity after sparge (before extract addition) and after extract addition pre-boil and it was spot on the estimated gravity into the boiler. I set the top off into the fermenter at 2.5 gallons expecting to get 2.5 gallons of wort post boil. I ended up having to top up with 3 gallons meaning i evaporated off more than expected? is this the reason I missed the O.G.? I am trying to figure out what value to tweak in order to get a good estimate since there are so many to change. the ideal thing would be to change one at a time correct? I think beer smith is great but its got a steep learning curve, and sorry for the long post just trying to post as much info as possible.

Thanks Guys

bump hoping some smart people will read this post and maybe offer some insight, or maybe ask me about some piece of information I may have missed. I am also aware that this could possibly be a user error measurement wise!

I have been using beersmith for over a year, and sometimes I have this same problem. Evaporation rate has a lot of variables, ie..relative humidity, diameter of your kettle, etc. I have my evaporation rate set at 15% for a 10gal 16in diameter SS kettle. Sometimes I come out ok, sometimes I need to top up the fermenter. I've learned to live with it.