No, you are mathematically correct in your statement, I did drop the minus in front of the Y. What I meant is, it should have been expressed as

Vtt x 0.667 + Y = Vtt x 0.635

1.14v x 0.667 + Y = 1.14v x 0.635
0.760v + Y = 0.724v
Y = 0.724v - 0.760v
Y = -0.036v
Y = -36 mv

But I expressed it deliberately incorrectly as

Vtt x 0.667 - Y = Vtt x 0.635

1.14v x 0.667 - Y = 1.14v x 0.635
0.760v - Y = 0.724v
Y = 0.724v - 0.760v
Y = -0.036v
Y = -36 mv

I did it like that initially in an attempt to make it clear that when setting up for a 45nm cpu, you must put a negative number (eg: -40mv) for the 0.667x multiplier Y value. However I think it would be better to express it as I have now, which is using a '+Y' in each of the examples, and it is now mathematically correct. Thanks for the feedback, it was clear what I was doing & why in my mind, but it's good to know that it would be better to just express it mathematically correctly rather than the way I had it initially.

Here's the corrected example with full working:

Vtt x 0.667 + Y = Vtt x 0.635

1.14v x 0.667 + Y = 1.14v x 0.635
0.760v + Y = 0.724v

subtract 0.760 from both sides of the equation

Y = 0.724v - 0.760v

Y = -0.036v
Y = -36 mv

Make sense now?