An index card is 3x5" (according to the package I have in front of me. I'd like to leave at least a half inch border so my 16 becomes 4" and the 9 then becomes ( 4 / 16 ) x 9 = 2.25" ( 2.25 / 2 = 1.125" from center top to bottom). You can use the same math on any sized card. The 16 x 9 is a ratio, so it's 8th grade-ish math (if you remember that stuff - I do, I'm a geek).
x / 16 = y / 9 (/ is divided by)
Since we have X in this case 4"
4/16 = .25
Multiply both sides of the equation by 9 to get rid of it from the Y side and you have your total height.
.25 x 9 = y
2.25 = y
Find the center of your card, and you want half of that distance each way.
2.25 / 2 = 1.125 ( 1-1/8")
The math is the same in metric for all you foreigners and sciency folks
Then you can mark your thirds lines if you like to help visualize framing, just divide the height by 3 and the width by 3 and make marks along all 4 sides of the hole you've cut out (I used an xacto knife and cutting mat to cut the hole out).
Have fun!