For self insured plans, how are the monthly rates (accruals) calculated?
I know that the insurance provider would develop the administration and stop loss fee however for the monthly rate required to cover claims (and charged to employee and employer) is this developed by the insurance provider, or by the company or companies broker?

Depends. You'll just have to ask to see who will do it. I've seen times where TPA's won't calculate them unless they are contracted to perform COBRA services, since arriving at COBRA rates is essentially what you are talking about. In almost all cases, I would think that the broker would do this for you. As Plan Sponsor, the client/employer should ultimately sign off on the rates if they are to be used as COBRA rates. Regardless who does it, COBRA rates are typically be based on claims utilization by enrollment category (single, EE +Sp, EE + Child, etc.) over the past 12 months, then trend added, then often blended with previous year rates so as to soften the blow of any spikes.

If you use the the insurance company as the administrator they will set all numbers. If you use a TPA, which is my recommendation the stop-loss carrier will set the rates for the premium and the factors for the claim liability. Useally based on prior claims history of the company claims.

The company could calculate them. We do our own. Seems every company I am aware of who self insures does it a little.