Getting the ratios of the materials correct consistently is the secret to good cement render or bricklayers mortar.
The general rule for composite mortar is 6:1:1. Now to mix this on site requires a mix that is 3 times the size. 18:3:3. This will fill a generic trade wheel barrow about 40 mm from the top. This full barrow can weigh in excess 150 kg.
Now a standard cement bag weighs 20 kg and consists of 16 litres of loose cement. Or four x 4 litre containers which commonly hold things like bondcrete etc.
Another common measuring bucket is the 15 litre texture bucket. If you place 3 shovels of sand into the bucket you will find that it fills the bucket not quite to the top. This is a variable measure based on how heaped the sand is on the shovel, something to pay attention to. If the sand is dark yellow it will heap high on the shovel as opposed to light yellow which will lay flat on the shovel.
One bag of cement fits in a 15 litre bucket.
When mixing the 18 shovel mix you will require a full bag of cement per mix. The standard way to mix the mortar in the cement mixer is to add at least 25 litres of water and add 12 shovels of sand then add the required amount of lime. Let the lime mix thoroughly into the sand. Now add the 1 bag of cement and also let that mix thoroughly. The object here is to add water as you go to get the mortar to roll off the back of the cement mixer as it turns. When you have a pliable mix, then just add the remaining 6 shovels of sand.
A word about additives as I work on a lot of building that are in excess of 100 years old. You really don't need to use them if you have followed the above. Poor mortar joints in brick work can be put down to not wetting the bricks as required to aid in the bonding of the brick. This means that the brick sucks the moisture out of the mortar mix prematurely. When jointing (rolling) the mortar if it is left to late, all that will be achieved is a sandy finish to the mortar joint, instead of a polished joint. Sandfinish that is poorly floated will be porous and let the rainwater into the render before it is painted.
When using this mix in a bricklaying scenario you will want a pliable mix that is capable of holding the water longer. This mix would a lime rich mix. As opposed to the plasterers where he can shift a lot of his mortar onto the wall quicker but he does not want it to "hang" on the wall.
How strong is the above mix? Years ago I done the brickwork on a cottage and was a few bags short. I just done this mix to finish. Now when I came to fix the beads on for plastering I had to use concrete nails. As opposed to the premix I had been using I was able to use clouts like normal. A block of units that I rendered was above the tunnel and m2 mortar was used and that also required concrete nails