I have always told my students in the event of lack of POH data to use a 10% per degree of slope correction factor for the takeoff distance to 50 feet numbers for a downslope and 20 % per degree for an upslope
I got this from a literature review where the 10% number seemed to be the most common correction to factor takeoff distances by authors who examined this question.
It is unfortunate that many GA airplane manuals don’t provide this information for each model.