+1 on Beersmith and the free trial. I'm sure there are a couple websites with free calculators as well if you google for it.
Just keep a little DME on hand to bump up if you have a bad efficiency.
Biggest misses on efficiency are probably bad crush on the grain, improper temperature realization or maintenance, improper water amount, and improper stirring of the mash to distribute temperature.
I flubbed my efficiency first partial mash, but had documented everything. On my next one (all grain) I saw a very good improvement by reviewing my notes and adjusting for my previous errors.
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