Currently doing fence posts and some small concrete pads around the house so I need to self mix the concrete

I keep searching for this type of thing on the internet:

Here is a good example of what is required from Boral’s Site

From my learnings:
1 Cement 2 Sand 3 Aggregate (gravel) is too strong. 16bags (costly)
1 Cement 2.5 Sand 4 Aggregate is OK for fence posts. 13bags of cement per cubic metre.

The more cement the more costly of course so don’t over do it.

Note: These are 20kg bags of General Purpose cement.