As you say the checklist says "8-10% minimum---".There is no reason why you shouldn't let the engine accelerate on the starter to 12% or more before opening the start lever.The minimum is set because start fuel flow is (more or less) a fixed figure,and introduction of fuel before the minimum rpm means that with reduced airflow through the combustion chamber you run the risk of hung/hot starts.Letting the engine accelerate higher on the starter means more air flow (including cooling air),faster light off and generally quicker acceleration to stabilised idle.If you stick to the published procedure you can't go wrong.Engineers tend to let the starter do more of the initial work because we are maybe ? more mindful of the internal stresses involved in starts and starters are easier and cheaper to replace than engines! Cheers!