Unfortunately it's nearly impossible to judge when you're ready for Heroic from normal completions. My guild got to the point where we were completing normal in a matter of hours, but because we had so many people that were doing their damage on "sub-optimal" teams we had a lot of trouble making the transition. If you have a good number of people who have really invested in resistance (can be used effectively in p2, 3, or 4) you may have a lot less trouble than us.
Ultimately you're just gonna have to take the plunge and make a run at it to see where you are at. Are you stalling out in p3? You may need someone that can do that phase solo like we do. Are you not even making it that far? Probably need to have some people put some more effort into resistance. The gap between difficulties is big enough that it's probably going to require some trial and error unfortunately.
Another thing to keep in mind is that cLuke can solo p1 easily as well (zKylo and zSavage still can too, but they take more time and effort), so in the early days you should be aiming for phase 1 completion without anyone in guild losing any characters.