First thing I would do is download a benchmark program such as Unigine Heaven or Valley or 3DMark and run a benchmark.  This will tell you what your max clocks will be.  As you noticed, it downclocks at idle to save on power and reduce heat.  Once you have done that, we will call that out baseline.
 
Now in regards to the offsets, what these mean for example, is if you offset the core clock +20MHz, this will add that on top of the max boost clock you got under full load.  So if you boosted to 1150MHz, then at +20MHz you should be theoretically at 1170MHz.  Due to rounding and other factors, it can be slightly more of less than the true offset value.  To get your max overclock, it is a process of repeating the offset in small increments, running a benchmark to determine stability, if stable, repeat the process until your reach an instability.  Typically that means your screen will go black and will have a driver error.  Once you have reached your max, then back down maybe 20MHz to have a little buffer room from instability.
 
This process above would apply to your memory overclocking as well.  To help with the overclocking too, you can apply a small overvoltage.  If you go to the voltage section, you can apply an additional +38mV.  Don't worry, this should not cause any issues.
 
Hope this helps, any questions, feel free to ask.  Disclaimer, your results may vary, overclocking is typically safe and will not damage a card, but I am not responsible for anything that may go wrong.
 
Good luck!

I guess I meant to monitor the core and memory clocks in Precision X while running the benchmark.  Rereading my post, I guess I didn't make that the most clear.  AlphaNerd, when you run the benchmark, either log the output from Precision X, have it open on another screen, or open up the monitoring graphs (double click them on the bottom of the Precision X window) right after you run the benchmark to see what your boost clocks are.  Then perform the process of dialing in by raising the offsets in small increments.
 
Since you are running SLI, I am not sure if the process would be substantially different.  I guess it depends if you have them synced in or not in Precision X.

Memory clock offset would be the same process, but best to do core first and then do memory.  If you haven't added more voltage to your cards, I would probably do that first before starting to overclock.  Your "stock" boost should even be slightly higher with more voltage.  If you click the voltages button in the Precision X window, it will pop up the small window on the left of the picture.  Click on the overvoltage button and then an arrow will show on the bar on the left of that window.  Slide that arrow up should give an extra 38mV.  Once you do that, I would run the benchmarks and see what your baseline is then.  It probably will be slightly higher than your 1241MHz.  Then just increase offset in increments, test, repeat as needed.  Hope that helped.
 

Yes, you don't have to up the voltage, but it will help with the overclocking ability.   Pretty sure the increased voltage will be beneficial for both memory and core clocks.  Glad I could help!

So can someone tell me what my memory clock is? I haven't begun to try and overclock yet, this is what my card is reading stock clocks. I just can't figure out what the memory clock is set to.
 

Ahhh didn't even notice that there ! Thanks for that. I'm going to try and overclock a bit on the GPU now :)

So I raised my clock offset +20MHz & Mem clock offset +10MHz and the clocks stayed exactly the same.... Still 1215 and 3506

Yup i hit apply and all that jazz. I'll try another +20Mhz. Could it be because the b00st clock is 1215Mhz and in theory I'm only raising the actual standard clock of 1175?

My god the 770 is a beast of an overclocker!!

1300Mhz Core clock
8000Mhz Mem clock

Absolutely no voltage increase and 100% stable!

Thanks for all your help trabe3

Here is a pretty good overview:
 

GPU clock only increases in increments of +13.

Its not always exactly 13MHz.

EVGA_JacobF
Its not always exactly 13MHz.

Which is correct, but it may as well be 13. I find if I drop or increase it by 1 it does nothing to the clock.

How big should the increased increments be, 20MHz?