First off, you should charge whatever you can charge, as @Dheer says in comments. However, that's really not on topic here.
What is on topic is how the 'contractor pay' compares to 'salary pay', and how you can choose which will benefit you more financially.
You will incur about 15% tax for the self employment tax, which is the employer and employee side medicare and social security taxes. You currently pay half of that, so basically 7.8% is new.
Your health insurance will not be paid for in any way; if you're married and your spouse has health insurance through their employer, you may not notice this. If you're single or your spouse does not provide health insurance coverage, this will cost you - but not a percent of your income, just ... something. Look at your current insurance payment per month and multiply it by 5, that's probably what you'll add (most employers pay around 80%). Or ask your employer for your COBRA payment (the amount you'd pay to continue your insurance after you quit); that's the full payment with no employer subsidy. Remember you may end up spending even more on the open market, especially if the ACA is repealed in part or in full.
You won't get any paid vacation days, either. If you had 15 paid vacation days and 6 holidays (I hope you had at least this much!) then you had 21/260 = 8% vacation days. This may not matter to you, if you don't take vacation, but I'd consider this an 8% pay cut.
You're also going to miss disability (short AND long term) insurance payments and life insurance. Factor in how important this is to you and how much it costs to get this separately.
You won't get a 401(k) match. You also won't get a 401(k); you may or may not qualify for a traditional (or Roth) IRA. You would instead qualify for a Solo 401(k) or SEP IRA, but you'd have to set that up and might cost you some money depending on how much you have to put in it.
Let's put this in numbers.
You're making $75,000 a year as an employee. You make $100,000 as a contractor. Which is more?
- 5,737 (7.8% - medicare/ss)
- 2,000 (health insurance)
+ 1,875 (50% match on 5% 401k)
$ 69,138 total pretax income including 401(k)
- 15,500 (15.3% medicare/ss)*
- 10,000 (health insurance)
- 7,700 (20 vacation days)
- 500 (disability insurance premium)
- 50 (life insurance premium)
$ 66,250 total pretax income including 401(k)
- Half of this will be tax deductible
That's probably missing a few things and the estimates are pretty sketchy, but I'd say it's reasonable. So you're going to make about the same amount in take-home between the two - a salary of $75k or a contractor's wage of $100k. But of course the insurance premiums are not a percentage of income, so the +33% isn't fixed: $100k salary comparable is 28% or so higher, and $50k salary comparable is about half again higher. The 30% estimate is just a very broad estimate. I would recommend calculating your own numbers in order to determine how it will affect you.