Lookup or array

Search algorithms is a rather broad field in computer science. The two most common are linear and binary. Wiki has good articles on both

http://en.wikipedia.org/wiki/Linear_search
http://en.wikipedia.org/wiki/Binary_search_algorithm

For small amounts of data, linear is faster. For large amounts of data, binary is faster. I would guess that less than 50 would be considered small. Your platform may have library functions for search an array of values. I don't know what your level of programming experience. If you looking for some example code, here's a possible implementation of a linear search:

typedef struct
{
  int   tc;
  float vin;
} TCVIN;

/*Ascending by tc, descending by vin */
const TCVIN tcvin[] =
{
  {25,1.9000},
  {26,1.7700},
  {28,1.6500},
  {30,1.5100},
  {32,1.4000}
};

int lookup_tc(float key)
{
  int i;
  int imax = sizeof(tcvin)/sizeof(TCVIN);

  for(i=0; i<imax; i++)
    if(tcvin[i].vin < key)
      return(tcvin[i].tc); /*Found*/

  return(tcvin[imax-1].tc);/*Not found use last entry? */
}

For speed, you should probably avoid using floating point operations and use the original ADC fixed point integer values.