There are really two overlapping issues here, whether the mix will have enough O2 to keep you alive and give a high enough FO2 to get the advantages of breathing nitrox, and whether the mix will be safe to breath at the depth you will be using it.
While you are using a SCR you are always "breathing down the bag" to an FO2 of a bit less than the FO2 of the supply gas, so a lot of divers figure its safe to base the max depth/PPO2 limitation on the inspired mix rather than the tank mix. The catch is, if you have to purge the loop, or have one other of several possible malfunctions, you could find yourself breathing gas at the full tank FO2. So if you are using 40% at 130 feet you could find yourself breathing a PPO2 of 2, which is generally considered too high. So the depth limits for different mixes on a Drager are the same as for open circuit (though many stretch it a tad and use 1.6 rather than, say, 1.4 or 1.2).
A lot of SCR users seem cheat on this, figuring that they are unlikely to ever, even after purging, to breath the full tank FO2, and that even if they do, a few minutes exposure to a higher PPO2 is unlikely to produce immediate harm. However this is debatable logic.
On the other hand, the jet flows for the Dragers, as others have already explained here, were calculated very conservatively, and before oxy monitoring was available. Most Drager users now seem to make it pretty much standard practice, after they get experienced on the machine, to use a jet one size lower than Drager recommends - for example, using 40% with the 50% orifice - to improve duration and reduce bubbling.
