was or were, I always get this wrong...

[littlefrog]

"Twenty percent of staff were American"

or

"Twenty percent of staff was American"

I think it's the first, it certainly sounds better (to me), but Word tells me it should be was...

Which is it?

Word is wrong. I think if the sentence were(!) "Its staff was American" then it would be OK, as 'staff' in this case is singular, but in your case you are talking about a number of individual staff so you need the plural.

On the other hand "If I was American" is wrong, as you are using the conditional case.

I think it is was becos 20 % is not a number but an amount.
Think of 100% of my test was correct/is correct.

Mmm. Is definitely NOT conditional, this is in the past tense.

I thought the issue related to the staff bit (singular noun for plural stuff), not the 20% bit.

So you might say
50% of the fleet (of ships) was sunk
But
50% of ships were sunk

On that logic it's definitely was not were, but it sounds terrible!

Grammatical geniuses (genii?) come and help!

Definitely not a grammar person - shouldn't really be on this board but I think it is either:

"Twenty percent of staff were American"

or

"Twenty percent of the staff was American"

I agree with littlefrog that it's to do with the nature of the word "staff". Strictly speaking, "staff" refers to one group (like "the workforce") so takes the 3rd person singular, rather than plural.

However, I think it looks hideous and I always change it to the plural!