Calculated Items that include other calculated items in the estimate

We need to calculate sales tax costs (when we buy materials for non-exempt end users), sales commission, and other things that should reference in ALL items in the estimate, including calculated items. We currently use calcItems for things like estimated freight, misc installation materials, travel and living,, etc. When calculating (e.g.,) Sales tax costs and sales commissions, etc., this needs to include everything. I understand the possibility of having an infinite calculation loop. So make a class of calculated items that are made to be applied as a "subtotal", and superior class that calculates based on that subtotal. If I could attach an example table I would...but....