How much will spine increase by cutting an Easton Axis 400 from 30" to 29.5"?

I always cut 1/4" at a time..it's really easy to cut em' too short and then your...

Trimming 1/4" at a time from the test arrow is good advice.

Yep another vote for 1/4 at a time.

I just got done doing a lot of tuning with AXIS traditional. if you are close at 30" a 1/4 will make a good bit of difference (well it did for me) if you're still hunting for that close tune and not fine tuning than 1/4" is a good cutting length.   

It's easier for me to answer this question by comparison to another method of changing spine, than to answer directly.  That's because you're changing dynamic spine when you cut the arrow, which is difficult for the typical home arrow mechanic to measure directly, unlike static spine, which stays the same after cutting the arrow.  My experience is that cutting 1/2" off a 30" arrow has about the same effect on spine as reducing the point weight from 200 grains to 150 grains.  I have also found that the effect is proportional; in other words, cutting 1/2" off a 29" arrow has more effect on spine than cutting 1/2" off a 30" arrow, and reducing the point weight from 150 grains to 100 grains has more effect on spine than reducing the weight from 200 grains to 150 grains.  My conclusions are based on my own observations of bare shaft flight and are subjective, as I have no way to measure dynamic spine directly. For that reason, YMMV.