What about this concept (pseudo code...)?
if(depth < min_linear_hight){
switch( depth )
case 1:
//one inch volume code;
break;
case 2:
//two inch volume code;
break;
else if((depth >= min_linear_hight) && depth <= max_linear_hight)
{
//linear conversion code
}
else{ // near the top....
switch(depth)
case 42:
break;
case 43:
break;
case 44:
break;
}
}