The power levels available in microwave ovens vary greatly from around 500 watts in low power ovens, all the way up to 3000 watts for some Commercial models. So microwave cooking times are necessarily imprecise. Use the timings in microwave cooking recipes as a guide only, and use your judgment, just like you do when cooking on the hob. Remember also that it is better to give your food another burst in the microwave if it isnt quite done that to have it cooked to a frazzle because you over cooked it first time. Think also about the ambient temperature in the kitchen. if you are cooking something for a short time only then it will make quite a difference to cook time if you have just taken the item from the fridge compared to something that may have been standing in the kitchen. There could already be a difference in temperature of 5 – 15 degrees.