Whether you adjust the DC depends on your goals with the campaign; namely if you want it grittier, tougher to recover. If you subtracted level simply to flatten the playing field and want Medicine to remain about as effective as normal, then subtract the level at which one gets Expert/Master/Legendary levels of skill (so -3 (for most classes!), -7, and -15). Normally the difficulty would decrease as the PCs level past that, so instead they'll remain that difficult until the PCs get their proficiency even higher (though items and stats can chip away at it too).