I don´t how which is the right information to calculate.
Could you take a look to the following link please:
http://www.conrad.de/ce/de/product/154085/OPTOKOPPLER-SFH610A-1-VIS/SHOP_AREA_14738&promotionareaSearchDetail=005
Under "Dokumente&Downloads" you´ll find the datasheet.
This would be very nice!