For Cell F6, instead of the massive cluster of comparisons, you could simplify it by using VLOOKUP. The format for the VLOOKUP command is:
VLOOKUP(cell with value, upper left of cell array with the lookup value and the result:lower right of cell array, number of columns to count over)
The data array will have the value to be looked up in the left-hand column, and the result in one of the columns to its right.
So for your spreadsheet the expression would be:
=VLOOKUP(C1,A3:B12,2)
This tells the VLOOKUP command to grab the value that is in cell C1, look for that value in the first column of cell group A3:B12, and upon finding the value report what is in the second column next to that value.
Now if you want to error trap it a bit, you could do:
=if(OR(C1<MIN(B3:B12),C1>MAX(B3:B12)),"Invalid value",VLOOKUP(ROUNDDOWN(C1/1,0),A3:B12,2))
This first makes sure that the value you put in is not less than 1 (or whatever the minimum is in the table) or greater than 10 (the current max in the table), and if you put a non-integer value it rounds it down to the nearest integer before looking up in the table.