a used oxycon doesn't deliver 100% pure oxygen if i have my oxycons on my Alpha i can only use 4 lpm to have some purity but the Alpha is a low pressure torch and uses more lpm! so you need more oxycons for better performance.
i used to use 2 of 5lpm for my boro but needed 3 to give it more purity! you have to because if there is not enough flow and you put the outerfire on you get carbon build up! but two will work if you only use the centre fire
why don't you use an bravo? i hope this helps