Cutting down a steerer tube...

Is the headtube length on old and new bikes exactly the same? Also, if it's a different headset then the stack heights could vary and require a different steerer length. I would build the new bike up with a fair allowance of extra steerer; measure the bar position; ride it for a bit to make sure it's right; and only then cut the steerer, by marking around the top of the stem (and then adding a mm or so to allow for a small spacer sitting on top of the stem).

Thanks for replying Regal.

The head tube on the new frame is 15mm shorter. The new headset is 2mm shorter too. However the new fork is 7mm longer and I'm using the same stem.

I guess what I wanted to check was that because I want my bars to end up in exactly the same position as before I will want the new fork to measure (mid axle to top of steerer) the same as the old fork. Make sense?

If I'm off on this ill play safe and cut longer than I need to for now.