Vs0 * 1.3
Remember that the Vs0 and Vs1 speeds in the book are given for gross weight, so we typically fly the approach a little fast anyway, because we usually aren't at gross when training in a 172. The only time I land a 172 with less than 30 degrees of flap is if there is a gusty wind, and then I add no more than half the gust factor to my speed. Trim for the final approach speed! I want to put no pressure on the yoke until I level off just above the runway and hold it off until the main wheels touch.
My point, overall, is not to overthink it. I am used to flying solo or with one passenger/instructor in the front seat, but recently I got to take an aunt and uncle for a ride in a Cherokee. I didn't even think about the effect on CG from having someone in the back seat. I just flew the plane the same as usual, got down low, held it off, and touched down very gently. I can also sense the difference in flying the Citabria without an instructor in the back seat, but again, I don't fly it any differently.