This can depend greatly on the aggregates you are using along with the block strength and finished texture you are trying to obtain. It also has to do with the quality standard you may be experiencing in your country. For example, in the USA we make a very tight smooth textured texture block because we use the block as a finished exterior wall unit. The most common ratio of cement to aggregates to make a normal height CMU building block is 1 to 10 or 1 to 11 Cement to aggregates ratio. When making a high strength structure block, the average is a 1 to 9 Cement ratio.
In other parts of the world it is not required to make a smooth textured block because the blocks are all stucco finished after being laid in a wall. That means the blocks producers are able to use larger sized aggregates which requires less percentage of cement in the mix. I have seen some of my international customers make very good products with as little as a 1 to 13 Cement ratio.
My theory is to always make the best product you possibly can. Using the smallest amount of cement possible could cost you losing existing and future sales in your business. I have seen this happen many times with companies I’ve studied that did not even use my equipment for manufacturing. Making too cheap of block with poor quality can make you lose customer and it is hard to regain the trust and get the customers back buying from you even if you are the low cost leader in the market.