Three part interpolation

[Jeeves8449]

I have a table representing three variables (d/t, L/d, and A). I have calculated values of d/t and L/d, and I need to find the corresponding value of A. My calculated value of d/T = 6.89 and L/d = 49.38. Below is the portion of my table.

d/T L/d A
5 10.00 4.59E-02
5 30.00 4.54E-02
5 50.00 4.53E-02
6 0.05 8.37E-02
6 1.20 8.37E-02
6 1.60 5.84E-02
6 2.00 4.69E-02
6 2.40 4.11E-02
6 3.00 3.69E-02
6 4.00 3.41E-02
6 5.00 3.29E-02
6 7.00 3.20E-02
6 10.00 3.16E-02
6 30.00 3.12E-02
6 50.00 3.12E-02
8 0.05 9.68E-02
8 0.74 9.68E-02
8 0.80 8.75E-02
8 1.00 6.60E-02
8 1.60 3.72E-02
8 2.00 2.85E-02
8 2.40 2.42E-02
8 3.00 2.12E-02
8 4.00 1.92E-02
8 5.00 1.84E-02
8 7.00 1.79E-02
8 10.00 1.76E-02
8 20.00 1.74E-02
8 50.00 1.74E-02
10 0.05 9.64E-02
10 0.56 9.64E-02
10 0.70 7.20E-02

Doing this by hand, I have to interpolate 3 times.

1. For d/T = 6: L/d = 30, A = 3.12E-02; L/d = 50, A = 3.12E-02; therefore at d/T = 6 and L/d = 49.38, A = 3.12E-02
2. For d/T = 8: L/d = 20, A = 1.74E-02; L/d = 50, A = 1.74E-02; therefore at d/T = 8 and L/d = 49.38, A = 1.74E-02
3. d/T = 6, A=3.12E-02 and d/T = 8, A = 1.74E-02, therefore at d/T = 6.89, A = 2.56E-02

I created a separate column for the tabulated values of d/T and named it d_T. Then used the formulas =INDEX(d_T,MATCH($L$3,d_T,1),1) and =INDEX(d_T,(MATCH($L$3,d_T,1)+1),1) in cells L5 and L6 where L3 is the calculated value of d/T (6.89). I can't figure out how to get the L/d's on either side of the calculated value (30 and 50, 20 and 50) in order to then index / match to get corresponding A's for interpolation. I know in this case the first 2 interpolations do not require calculation, but I have to do this 22 times with different values of d/T and L/d that will require calculated interpolation.

Any thoughts?