I have a standard Orange Rubbermaid 10gal mash tun with a SS false bottom, and for me BeerSmith nails the strike water temp every single time for years running. I’m able to get all my house beers, and new beers, within a single degree consistently. Frankly, it’s kind of eerie, because my batch sizes are all over the place. To me the strike temp calc. function is the one perfect shining example of a reason to purchase BeerSmith.
Now.., you have to standardize your process. I make sure to rinse my mash tun with my hottest tap water 10min before strike. If I do it 1 hour before strike, it’ll be a couple degrees off. If I do not stir my standard amount of time, I’ll be off a degree. Etc. I loose 8° in an hour with it closed tight.
This is my equipment setup info:
Mash Tun Vol. = 10
Mash Tun Weight = 11
Mash Tun Specific Heat = .3
Lauter Tun Deadspace = .625 (actual)
This is the thing – what is your [Mash Profile] setup as? That is where you set your temperature, and all your step check boxes must correspond to what you’re doing.
To me the only confusing thing about BeerSmith is that you can change the same value in a multitude of places, and there are excellent reasons for that, but it can be confusing. In some forms you can’t easily see what fields are manually input vs. calced too. It took me quiet a while to get it all squared away (and I’m semi literate technically.)
(However, full disclosure, what I cannot fix in BeerSmith, and I’ve just always worked around it out of laziness, is the fact that I get about 20min of extra volume wort EVERY TIME,

so I’ve got my own axe to grind. (I’ll create a separate thread for that, to not confuse this one.)
I suspect your dealio is in the Mash Setup (Note: if you change values in the template, that is different than changing values in the recipe copy. You'll have to re-select the modified Mash Profile template from within the recipe.)