OP,

you have the luxury of having a replenishing bankroll from the sounds of it. therefore, i would start at the lowest stakes i would take seriously. if that is .10/.25c blinds, start there. if its .05/.10 blinds, start there. it really doesnt matter......if you can keep replenishing your "bankroll." have fun, learn the game, and study where you may be going wrong so you continue to improve.

no one can tell you where you SHOULD start. we all have different values we place on specific amounts of money. we all have different adversities to risk/gambling/running out of money and starting over. so, make up your own mind.

however, remember that the higher you start, the harder you make it on yourself to learn fast. the competition, especially today, is tougher than ever online. starting too high may actually stunt your growth with the concepts you pick up on.

recommendation: (and take this with a grain of salt) start at about 5NL, play about 10k hands, if you are winning for more than a 5 ptbb/100, move up. if not, start to review your hands. keep moving up after 10k intervals until you can't comfortably replenish your bankroll if it disappears. and, dont be afraid, due to pride, to move back down a level if things dont go so well.

*the above recommendation is dramatically different than i would suggest to someone that didnt mention they are a "successful professional" and could afford "significantly more" than the $50 OP is starting with.

(i guess that was post 4k...lol. maybe it should have been deeper, but i think it's fitting it was in the BC.....where it all began. make use of it, as i'm sure it's of the same quality of all my others....lol.)