Each diamond is 0, 10, 20, etc., Break the diamonds (inbetween) up into tenths, the pocket you are aiming at is 0.
Half way between 0 and 10 is 5. Or half way between 10 and 20 is 15.
After this is is easy to visualize 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10.
0 being the pocket (imaginary diamond) and 10 being the first diamond.
So shooting a short rail bank or kick into the corner on your left...
The corner pocket is 0, then first diamond is 10, next diamond 20, next 30.
Same thing with diamonds on other side of table.
Shooting from 20 (the second diamond on your side), you would aim at 10 (the first diamond on the other side).
The "inbetween" numbers work too! From 16 on your side, you would aim at 8 on the other side. Or from 26 on your side, you would aim at 13 on the other side.
If shooting into the side pocket, switch the numbering around with 0 being the side pocket.
If you have your own table, you can put masking tape behind the diamonds and number 0 (corner pocket), 1, 2, 3, .... 38, 39, 40 (side pocket). (Or other way around.)
Don't leave the masking tape on too long as it will be hard to remove.