I use a builders bucket too, my electric belle takes a full wheel barrow full. Roughly half a bag of cement is 2/3 of a bucket. So 5 x 2/3 of a builders bucket makes a barrow full, or it did today when i was using it, to concrete in a couple of gate posts.
Was looking on a bag of master crete, i have another 25 lying waiting to do a job.
4 to 1 or 321 mix is what it says on the packet. Let it mix dry before adding any water.