1) Perhaps it is so nobody accuses them of directly using the NR implementation without attribution. That value is just an empirical guess, so there is nothing too magical about 0.1 or 0.4 (better be less than 1). Since you have found where this value is used (not too hard), I suggest you do the experiment by changing it to 0.1 or 0.2 or 0.5 and see if it makes a difference. Let me know, I haven't tried it. When I implemented the NR routine in LV I used 0.1 and it worked, and when I use the LV routine with 0.4 it still works.
2) If you select f(x,a) is a VI to use a sub-VI to calculate the function values you can also return the analytic derivatives. This can save time and reduce round-off error, and I whole-heartedly recommend it when feasible.
