I have never designed a boat, but I have fiddled with our boat, tought about it and tried some stuff. There are practices and a firm scientific basis for where to place underwater appendages. It is not really that difficult once you have decided on a rig and sailplan. You calculate the center of effort developed by the sails, and design the underwater appendages to suit this. Or start the other way around.

If you sail mostly with main and spi solo on a two person boat, you will probably need to change mast rake to balance the sailarea with the underwater area (CoE must balance CoLR) when you take off or put on the jib. So you will have less mast rake sailing one up and more mast rake when sailing with the jib. You can feel it quite easily when sailing if the sails CoE dont balance the CoLR, you will either have lee helm or windward helm. When the helm is neutral when sailing upwind, your boat is in balance.