Where do you plan to use the ramp? Is it on an elevated coop so they can enter the coop or inside a coop so they can get to nests and/or roosts? Are you talking about older birds or newly hatched chicks with a broody hen? Do you have chickens like Silkies or Frizzles that can't fly. If you are posting a photo could you please give height dimensions.
Most chickens don't really need a ramp, they can fly up and down if they have enough room. The problem with that is that they have to want to. If they don't want to they can be more stubborn than a two year old toddler about potty training. As always there are exceptions and limits. I've seen two week old chicks fly over three feet (1 meter) when the broody hen tells them to, but they listen to a broody hen a lot better than they listen to me. I wonder if a 2 year old toddler would? Hm.....
Sometimes inside the coop they may not have enough room to spread their wings and fly. Ramps are often a good idea there but can take up a lot of room. Chickens can also hop really well. An alternative to a ramp might be a sloped ladder so they can hop from rung to rung or if the distance up isn't too bad just a stepping stone. As always plenty of options.
What material will you use for the ramp? Something really slick like plastic or painted wood or something they can grip better? It's usually a good idea to put slats across the ramp every so often to help them get a grip. Usually a cross slat every foot is enough but you can go a bit tighter if you want to.
How flat to make a ramp is a matter of opinion. If they can get a good grip 45 degrees could work. If I were doing it I'd probably go for a 30 to 35 degree from horizontal ramp.
I don't use ramps. My coop is big enough they have plenty of room to fly up to my 5 feet (1.5 meters) high roosts and 3 feet (1 meter) high nests. At the entrance to my elevated grow-out coop I build a series of steps maybe 8 inches ( 20 cm) apart vertical and horizontal.