Yes LGF that makes sense. Magura master pistons for rim brakes are larger than disc master pistons (mine are anyway) so the Maguras would displace more fluid for an equal piston stroke. So when using a Magura rim brake master cylinder with a disc caliper the system is pushing too much fluid and in addition to this the difference in cross sectional area of the master and slave cylinders is less than intended, which means mechanical advantage is reduced, more hand pressure therefore required at the lever to achieve a given braking force. The rider squeezes hard on the lever, it feels solid but the bike doesn't stop as well.
The OP want's to do the opposite (disc master, Magura slave) so in theory should experience the opposite; The too-small master not displacing enough fluid to generate full travel of the Maguras as suggested by M_T_S. It might 'work' if the rear wheel is super true and the blocks are sitting very close to the rim, but even then it may need more lever travel than desired to apply the brake fully, but ultimately should brake fully with less lever pressure. So even if OP got this combination working, the bike would look balanced with it's matching levers but the travel, feel and required lever pressure of the front and rear brakes would not match.
It stands to reason that mixing disc masters and disc slaves of different brands is much more likely to succeed due to the similar design.