Have you got the means to run the linear actuator yet on the voltage you intend to use it on? You could always attach progressively heavier weights to it to see what it can comfortably lift. From there you can figure out how much and how fast you can theoretically lift with it in your bot with different arm lengths for your lifter. Then you can see what the actuator is capable of and decide if it is going to be suitable for you or decide if you want to find something else.