With Portfolio Margin, margin requirements are determined using a "risk-based" pricing model that calculates the largest potential loss of all positions in a product class or group across a range of underlying prices and volatilities. This model, known as the Theoretical Intermarket Margining System ("TIMS"), is applied each night to U.S. stocks, OCC stock and index options positions by the federally-chartered Options Clearing Corporation ("OCC") and is disseminated by the OCC to participating brokerage firms each night. The minimum margin requirement in a Portfolio Margin account is static during the day because the OCC only disseminates the TIMS parameter requirements once per day. However, Portfolio Margin compliance is updated by Interactive Brokers throughout the day based on the real-time price of the equity positions in the Portfolio Margin account.
You can find more information on Portfolio Margin on the IBKR website.
Given all of the above, the simplest way to determine the margin requirement for a given position is to use the Check Margin feature in the TWS. For additional information, including instructions for using this feature, see the Interactive Brokers Knowledge Base.