My thinking is that you are advanced enough of a brewer to do parti-gyle, you can figure out the basic math it would take to your efficiency.

In short though:

Your hydrometer reading can be equated to the number of gravity points per gallon. Its a simple converting: 1.050 on the hydrometer means you have 50 pts per gallon (pts/gal) of wort.

From here you can multiply your pts/gal by the volume of wort you have: In my example you have 50pt/gal. Say you have 2 gallons of it...that is 100 pts total (50x2).

There you have it. You can calculate the total number of points in each running of your partiglye. Adding them up with tell you your total number of points extracted from the grain.

To calculate efficiency: Divide what you got out of the grains by what the theoretical max of the grain is.

Each grain has a published ballpark theoretical maximum of pts/gallon. For example 2-row is around 34 pts/gallon. This means if you take 1 lb of grain and get 1 gallon of wort/running from it, the absolute best you can do it get 34 pts from it. The hydrometer should read 1.034.

What you do now is calcuate the max pts from your grist by each quantity of grain by its max potential (pts/gal), then adding them up. That is max number of gravity points.

So for a grist of 6 lbs of 2-row: that is 6*34= 204 pts. If this is the grist that you used in my example above to get that 1.050, then you had a 100/204 = 49% efficiency.

Hope this makes sense.