Not sure I can help you with the deep parameter. Perhaps you figure that out or supply more in depth information. You will need to calculate the radius of the arc you describe. Best to find you a simple to use calculator such as this one: http://www.handymath.com/cgi-bin/rad2.cgi. Once you calculate the radius you can establish the diameter of the needed circle. Draw a true circle in Corel and change the dimensions to match your derived diameter. Draw a rectangle to you length and height desired. With Snap to object on, select the mid point of the rectangle's upper line and move it to the top of the circle quadrant node. If your math is correct the lower rectangle corners should intersect the the arc. The above calculator will also supply the subtended arc angle.
Edit: I realized what you wanted by the term deep, I think. You can duplicate you existing circle. Then subtract the 6 inch different from the Radius of Arc given in the calculator, double that to get the new circumference and enter that for the new circle..
3618.Arc Calculations.cdr
See the attached CDR.
There is already a post on this at http://community.coreldraw.com/talk/coreldraw_community/f/29/t/49150#pi1484filter=all&pi1484scroll=false