How much do I add on top of the card's clock speeds do you know? I'm using EVGA so it's either plus or minus clock speeds and not set clock speed to x amount.
I have been using the card reviews at TechPowerUp.com for getting a good baseline on what the card can be OC'd up to.
http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_750_Ti/28.html (just find the card you have)
I am not exactly sure how they come to their settings, but they have put me right in the ballpark for both of the cards I have right now.
Their reviews are extensive, and the overclock info page has the GPU-Z screenshot of their overclock settings.
I have found that everyone using the +100/+500 terms varies so much based on the card you are using and if it comes out of the box with any OC settings is kind of useless unless you have that exact same model of card.
My MSI 750ti OC 2GB running at their recommended 1145 MHz GPU/1675 MHz Mem and for 3 of my 4 cards that works flawlessly (~295 khash/s). Card 3 seems to not be able to handle that, but I scaled it back a bit until it was stable and figure it is just not a perfect card, but haven't had time to delve any further into that.
I also have two MSI 750ti TF 2GB running at 1210 MHz GPU/1675 MHz Mem. These are hashing out faster than the OC model, and run a good 14 c cooler even at the higher speeds (~325 khash/s).
Unless you have the exact same model the other people's settings will drive you nuts not being able to match... it took me a good two weeks to understand why I couldn't match the settings others were using, but it was because my card was pre-oc'd and only needed another +31MHz on the GPU to get the same settings.
Hope that helps.