Incorporation umbrella for 1099 consultants

[aGCseeker]

Hi,

After 1 1/2 years of GC, I quit my consulting company, and found a contract for myself and negotiated the rates. The consulting company offers standard 10% extra on W-2 for corp-to-corp. I was planning on incorporating, however one of their conditions in contract based on 1986 Tax reform act, is that my corp have no more than 80% of income through them alone. They recommended using a yurcor umbrella (yurcor.com - similar to freeagent.com in earlier boom days).

What I wanted to know is:
1> Has anybody used yurcor services and can tell me their experience with such?
2> Do I have to estimate business expenses in advance, and what happens if I don't use enough of expenses?
3> Is the formula as simple as:
earnings - actual business expenses - yurcor service charges = take home gross-income?

I am going to pose the same questions to them, but I appreciate any other feedback asap, since I have to sign a contract using whatever corp in next 2 days.

Thanks,
Vinay.