Pan F+ is a rather problematic film in that it develops contrast very quickly. You are right in wanting to reduce the EI. Whenever I use this film I expose at EI of 32 and develop in D-23 1+1. You are using a low contrast developer with a high contrast film to tame it. This film/developer combo is excellant. D-23 is very easy to make and very economical since it contains only 2 ingredients. If you don't have a scale there are teaspoon versions on the web.