Lump sum distribution of non-qualified pension plans

Your title says "non-qualified", so we will assume this is a payment from the company rather than a payment from a (qualified) plan.  

Without knowing any details beyond your statement, it seems likely the 20% is incorrect.  That percent applies to a payment (usually a lump sum) that is rollable.  It is very unlikely that any payment from a non-qualified plan is rollable to an IRA.  BTW, you used the word "taxable", but I think you mean "tax withholding rate"; your actual tax rate will be based on all your taxable income, deductions, etc.

Are you receiving a W-2 form each year for IRS/tax reporting?  If so, there might be a default withholding rate but there is no mandatory rate; you can submit a W-4 to elect your withholding status (not a W-4P, which applies to a payment from a qualified plan).  I'm not a tax expert so I hope other readers will chime in here to confirm or correct my statement(s).

Following up on david rigby's response, your nonqualified deferred compensation plan payments are considered wages from your former employer, subject to wage withholding (Form W-4) and reported to you for tax purposes on Form W-2.

I am extremely suspicious about your question. If this were a non-qualified pension, you should have been either sophisticated enough or rich enough to be able to understand the tax consequences or to hire someone to advise you about the tax consequences. I think you have a qualified plan distribution, but am extremely skeptical and cynical. There is nothing wrong with you or the question itself, but a proper characterization is important for people to give you a proper helpful response.