The depth of your leg press will influence how many muscle fibres are recruited during the leg press. Aiming to get your knees to a minimum of 90 degrees will ensure the legs are sufficiently challenged, however the deeper you go the more strength you will build at end range motion. A good approach is to perform some sets at a weight that allows you to reach 90 degrees, then dropping the weight to increase the depth. However, you should only go to a depth that allows you to maintain a flat back – this will depend on your hip and ankle mobility.