Twenty percent of your bodyweight is an approximation – the amount of weight that you need to be able to lift depends on your needs.
What do you need to be strong enough to lift, to make your life easy? Do you lift big bags of dog food, kitty litter, cases of wine, or a cooler of food out of the trunk of your car? How much do these items weigh? Is it a struggle, or can you manage these lifts easily?
If you travel frequently, consider how much your suitcase weighs, as you need to be able to lift it off the conveyor belt without injuring yourself. The conveyor belt adds a rotational force to the lift as well, making this activity risky if you are not strong enough.
Lifting technique is important to preventing injury, and taking advantage of your body weight to help with the lift can make the lift far easier. Practicing deadlifts in the gym can help teach you the technique.
First bring the object as close to you as possible. The key to this movement is keeping the spine neutral, arms sucked into their sockets. Reach the buttocks back to the point one feels a slight stretch in the back of the legs, then grasp the object, keeping it close to the body.
Anchor the heels to the floor, feeling the connection up the back-leg line to the sit-bones of the pelvis, and use that line-of-pull like a guy-line on the top of a tent pole, to pull the wheel of the pelvis around until you are upright.
There is no efforting by the low back to lift the object. Although the spinal muscles will be active, the effort comes from the back of the legs. By reaching back into the back-leg line in order to counteract the weight of the object being lifted, the body almost acts like a teeter-totter. Get enough body-weight behind, and the object is lifted quite easily.
Knees are slightly bent, shins remain vertical, and the butt reaches back to the point of slight stretch. Spine remains neutral, arms stay sucked into the arm sockets, and as the pelvis-wheel rolls around to upright, it also moves forward to take its place under the ribs again.
It feels like a strange contradiction. Even though the weight is heavy and effort is required to lift it, when the mechanics are right there is an ease about the movement. There is effort, but the movement flows and just feels right.
Remember that training movements is far more important and useful for good function than training muscles. Getting stronger by getting good at these different movement patterns will translate into more ability to do what you want day-to-day.