The increase of Magenta to make the paper more contrasty gives only slightly longer times - if you look for the shadows getting real black. If you have a test strip and choose the first time with real black in the perforation area, then this time is nearly the same if you add 30% Magenta.
The first thing I try to find out with test strips is this minmal time for maximum black. Then I control the highlights with the help of the contrast, usually I need more Magenta. The strip with maximum black and nearly perfect highlights is the base of evaluating my exponation plan. It is easy to understand and works fine.