peter s,
Sorry, I found your OP a bit confusing, I thought your Mint was made for 250 mm. Never-the-less, the Mint arc is specific to the P2S. A Mint arc for 290 P2S will have a flatter arc than one for 250. The null points have the same distance from center in each case but the arc between them cover a different number of degrees. Think of an arm 10 feet long. The alignment grid lines at the outer null point would be nearly parallel to the alignment grid lines on the inner null point. On a ten inch arm they are not. The null points are not in line with the spindle but on an arc that overhangs the spindle. With proper magnification - at least 16x or so for the Mint- the problem will become obvious on your smaller scale.
Sorry, I found your OP a bit confusing, I thought your Mint was made for 250 mm. Never-the-less, the Mint arc is specific to the P2S. A Mint arc for 290 P2S will have a flatter arc than one for 250. The null points have the same distance from center in each case but the arc between them cover a different number of degrees. Think of an arm 10 feet long. The alignment grid lines at the outer null point would be nearly parallel to the alignment grid lines on the inner null point. On a ten inch arm they are not. The null points are not in line with the spindle but on an arc that overhangs the spindle. With proper magnification - at least 16x or so for the Mint- the problem will become obvious on your smaller scale.