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The oxide capacitance is actually available in the documents as far as I remember, and if I remember correctly it should be about 10 fF/um2 for thin oxide and about 5 for thick oxide devices. You better double check though.
But a few notes:
1- These numbers do not include anything else but the active area. So your total cap density is going to be lower.
2- Don't trust these numbers, they're just there to give you an idea. Process causes them to vary drastically. You better just run a simulation for measuring the capacitance.
Edit: I think the numbers I remember are for the cap in inversion. I'm sorry, apparently I didn't read your question properly. But it should be a similar number at its highest point. Also this made me question, under what bias conditions are you looking for its capacitance?