Thank you for your feedback. DreamLine does not recommend the use of screws to secure our base to the studs. This can result in spider cracking of the base, and ultimately could lead to leaking. Installing the base in this way and without the bed of mortar will void your warranty on the product and can compromise the installation. We recommend correcting the installation. The amount of mortar that you would need varies depending on how out of plumb your floor is so we cannot give a recommended amount. Most mortar companies have a table or calculator on their site to help you determine how much you will need based on the size of the base. You can always reach out to DreamLine's Customer Support Team with installation questions. DreamLine's Customer Support Team can be reached by phone at 866-731-8378 or by email at support@dreamline.com. We are available M-F from 8am-6pm EST. Thank you for choosing DreamLine.