You will be able to calculate the actual UV power at the surface of the print being cured from your figure of 2.5mw/sr. You must take into account the distance from the LED to the surface and must always assume some losses. (At least 30% being a reasonable starting point)
What is your reflector made from? UV reflection is very different to visible light reflection in terms of efficient surfaces for reflection (a glass mirror is poor compared to an aluminium reflector for example)
This link gives you the formulas to work out the power at the print surface. Aim for a minimum of 5mw/cm2 for a post cure in a reasonable time https://docs-apac.rs-online.com/webdocs/0321/0900766b803218da.pdf