You need to assure that you earn (after taxes / deductions, with the benefits added) at least as much as you did as freelancer or it makes (at least financially) no sense to switch.
From experience I can say, if a company is willing to pay a freelancer X amount per day, they'll be willing to do the same for an employee (usually including benefits, but negotiable).
If they want you really badly, you can even ask for more as an exclusive or signing bonus, I did.
Remember you might gain benefits like paid holidays / parental leave, reliable, constant income (at least usually) and better worker protections but you also lose a lot of freedom, tax deduction possibilities, and time flexibilities.
How about the fact that you're your own boss and any slave labour/overtime or whiny client you can chose to take on or kick to the curb by your on volition.
Try turning that into an amount per annum.
Benefits like healthcare plans are something you can have as a freelancer (though often at higher premiums), so are just another amount to include in the calculation.
Tax rates differ greatly between employee and freelancer, be vigilant.
Keep also always in mind:
permanent contracts, as nice as they sound, are never permanent and any fluke within the company or industry can render you unemployed.
If you want to go mathematical:
(no warranties, it's late and I'm just messing around)
Fh = your hourly freelance rate
Fa = your annual freelance rate
I = your actual, past annual average freelance income
P = your actual, past annual average freelance profit
B = employee benefits measured in money, kaching
Df = your taxes and deductions as freelancer
De = your taxes and deductions as employee
Wa = working days per year
Wh = hours per work day
S = your employee salary
Wa = 260
Wh = 8
Fa = Fh x Wh x Wa
S = Fa - B
P = I - Df
(S - De) >~ P
Calculating P is redundant since you can read it on your tax returns but meh, I included it for fun and a semblence of consistency / clarity ...
Also, some expenses like office, liability insurance or hardware / software are mostly not required for employees.
Oh and yes, if you relocate, account for cost of living differences, just like John R. Strohm said.