I’m struggling figuring out how you would do the following?

Work sheet data.

A1 =25 B1 = 40

C    D
1    25    30
2    25    40
3    27    41
4    28    42

What I want to do is lookup the values 25 & 40 (A1 & B1) from table C1 D4
Matching the value 25 in Column C & 40 in Column D & return the location in
the table to Cell A2?  If there is no matching value, were decimals come into
play, i.e. 25.8, then return the next largest from column C with its parent
value from D, Returned value = 2 or 3 if 25.5.

Sounds simple but I get lost very quick!
Any Help greatly appreciated.