How to count matched donations for GWWC pledge?

It mostly makes sense to me. Gift Aid has much the same purpose and ultimate effect in the UK as the US' tax deductions for charitable contributions, I think.

In the US, if I give $10,000, I will (often) be able to deduct a certain amount from my tax bill -- the amount depends on my marginal tax rate, but let's say $2,200 for this example. So the "cost" to me of directing $10,000 to the charity is $7,800.

In the UK, I can give $8,000, and the UK government will kick in an extra $2,000. I think that is seen as coming from my tax payment (in that if the charity gets more Gift Aid than you paid in tax, you're on the hook for the difference). So the "cost" to me of directing $10,000 to the charity is $8,000.

In both cases, the government covers ~ 20% of the donation, and I am only out 80% of the donated amount. The mechanics are just different. So unless you're going to require US donors to reduce their gift by the tax benefit for pledge purposes -- which would add complexity and be unpalatable to many people -- allowing Gift Aid to count seems necessary for equality of treatment.

I think that's a fairly democratic stance in the UK, because most pledgers will have enough income tax liability to fully benefit from the tax advantage. It is less so in the US, where low and many middle income taxpayers do not benefit from itemizing (especially after recent changes to the standard deduction). 

In contrast, allowing matches to count would call for very different sacrifices from pledgers based on a circumstance totally divorced from ability to donate -- whether one's employer matches. It creates a tier of 5% (or even 3.33%) pledgers.

I do, however, think that an 8% pledge target should be acceptable for US residents who do not itemize, or UK residents who cannot claim Gift Aid for some reason. Because the burden of donating is not reduced by the tax system for them, they are being asked for proportionally more sacrifice than their wealthier, itemizing countrymates, and for more than similarly situated people in the UK (and probably other countries). In other words, the 8% target would treat non-itemizing US residents / ineligible UK residents as though their donations were receiving UK Gift Aid.