You have to create your new screenmodes with picasso96mode.
p96mode has quite strange UI so it might not be easy to use it at first but luckily we have tutorials on teh net
try with this one
http://www.amigakit.co.uk/support/picasso96mode.php Even with a tutorial it might be tricky to create a screenmode, when i have created screenmodes for my tft's i have always had big troubles creating the monitors native screenmodes while resolutions below the native one has been easy. i have had to use the edit button and edit them while my monitors gives me that "out of frequency" message so i've been forced to have patience with them
