Texture of the biryani depends on how long you precooked the rice. If you feel rice is overcooked, cut back on the liquids (saffron milk+reserved water). When rice is undercooked, add more liquids. If your biryani itself is mushy and over cooked, it cannot be fixed. However, if the biryani is under cooked, immediately cover the pot and let it sit for few minutes. Rice will cook in the steam. If biryani is cold, sprinkle some water and cook on low flame for few minutes.