My steerer tube length should have 0 to do with what yours is. I'm 6'5 and have a larger bike, therefore a longer steerer tube.
1- insert new fork into head tube.
2- add spacers as needed.
3- mark location where stem should be.
4- remove fork and measure the steerer from crown to your mark. There ya go- that's how long it should be.
A steel steerer can hold more spacers than a carbon steerer.
30mm of spacers is consistently claimed, but conservative.
40 or 45 wouldn't make me think twice and I've seen that listed for some brand name forks too. It all depends on how you ride and how hard you are on the front end. One of my kids has a carbon road fork with 45mm of spacers...she is 70# nothing will happen.
There is not definitive point at which your fork's steered will snap.
Either way you are pretty screwed, if I'm reading your post correctly. You have 30mm of steerer above your head tube and in that space you need to squeeze the top of the headset and stem?