You can keep a minimum of 30% of the amount you charge for yourself, ideally on the net profit. That's a safe number that should cover some extra expenses or revisions you haven't planned, and your time for managing the projects.
The other thing to consider... Start a bit lower than $50/hr if you are paid $75/hr and raise the freelancer based on performance and how much energy they require you to manage them.
Some people need to be micro-managed and this means more work for you. Some people will work slower than you,some won't finish the projects or will require you to review their work because they're negligent or not attentive to details or let you down, etc. AND the time you'll spend finding your candidate will also cost you your own time; youmight end up re-doing whole projects that were already paid! You'll probably (certainly) try a few freelancers before finding a pearl and one you can count on. Working with another freelancer is unfortunately not as efficient as if you could simply clone yourself, and you need to calculate that loss in the percentage you'll decide to give them; count this as a global loss.
Your accountant won't really help you on this (eg.profit) but if you can find someone who manages employees, you could get good advice from that person.
Look at the market and average hourly rate, create a table of hourly rates based on experience and skills. Not all freelancers are worth the same and you can't pay a freshly out of school designer the high price since they will cost you a lot of money while you train them! A junior should get less than a senior, and if they have other useful skills(eg. web dev, prepress, SEO, marketing, editing, etc.) then that can be rewarded too.
For the taxes, you usually get a tax credit but you still need to count this as an expense when calculating what you should give to the freelancer.
