I do not think I fully understand what you need to calculate but if I remember correctly, there is a workaround to apply support hours to "custom" time calculations using Planned Start/Planned Finish/Planned Duration if you don't actually use these fields in production. See if it fits to accomplish what you need.
How to use the 'use support hours' is not really very clear. It appears you can only use it in conjunction with Service Level Agreements (SLA's). Both the implementation of SLA's and 'use support hours' in SD45 are about as clear as mud.