I plan on doing contact prints as well, how long should the exposure be with Multigrade paper by using the white light on the enlarger, any ideas or is it trial and error again?
Trial and error is best, there is too much variance in equipment and negatives to quote a good number.

FWIW, I prefer using the multi filter settings and rarely expose with white light for MG papers, for me, it makes testing and contrast adjusments easier, YMMV though.
I made the attached chart and taped it to the front of my enlarger as a grade starting point for my 4x5 LPL so that I didn't have to constantly wade through the table in the paper info sheet.