Regarding service levels. There is no standard, nor should there be one. Service levels are set to reflect business objectives and customer needs. Each business is different.
As for calculating peak levels from the average, Chris is right. There is no rule of thumb to do that. Some businesses see little difference throughout the day, whereas others have a distinct peak (or two) as well as daily and seasonal changes. Additionally, your ability to tolerate peaks depends on your target service level and agent utilization, which the Erlang calculator factors in the forecast.
If you have to solve this problem “now”, use the average as a starting point and monitor performance and utilization during the day and the week. A good resolution to do that is at least twice every hour. If necessary increase manpower for peak times (remembering that by doing so you decrease the overall utilization). Record call volume for long term trending.