I think that, initially, I would be more comfortable diving to depth from which I could emerge fast enough to not require breathing (for example, in case of a total equipment failure). What that maximum depth would be, considering that the ascend must be gradual ?
Some excellent comments so far. Once certified, Diver's are often comfortable with going no deeper than they can ascend on one breath (Emergency Swimming Ascent or CESA). Obviously this is a last resort because you dive with a Buddy who should be within touching distance (you're trained to access this breathing source). Some Divers also carry redundant gas, as a secondary source. Regardless, diving within an envelope of reaching the surface by performing a CESA is a great place to start. This should provide you with the level of confidence you seek. As you gain experience, your abilities (and those of your Buddy) will increase. At such time you may wish to progressively increase your depth and maintain confidence. To answer your question regarding depth, my OW Students are required to do an OW CESA from 50 FSW (regulator in mouth).