If you want the shorebombardment rule to be accurate, you should think about what role it played historically.
In WW2, most divisions had an artillery component. If you count the number of light, medium and heavy guns in divisions, you quickly discover there where ALOT of guns, and the armies used a lot of resources to have them there. Artillery where extremely important. When an inf attacks at 1 and defends at 2, that unit has alot of artillery as a part of the piece. Otherwise it would attack on 1 for each 3 units or something. In essence, artillery is important.
When you have a regular battle, you deploy some artillery at the front, and some on the sides, and some in the back. What is deployed where is dependent on the type of artillery and the type of terrain. When you do an amphibious invasion, you can deploy none of your artillery. You can have some marine landing vehicles with some small caliber guns and perhaps a few amphibious tanks. This is how the ships guns where used, they where used as a replacement for artillery.
Therefor, If I where to try to make an accurate Shore Bombardment rule, I wouls make it so that, in the first round, you could only attack with 8 units per supporting BB, 4 units per supporting CR and 1 unit per supporting DD