Well, you need a basic grasp of statistics if you want to do anything meaningful here.

First of all, the difference in level between 70 and 71 is not high enough to give statistically significant results (difference in conversion).

Secondly, 500 conversions on a total of 128 sailors doesn't have any statistical significance for each individual sailor, either.

A very important factor is the number of vets that are already on the sailor, in your examples much more important than any other factor. Your sailors have between 46 and 138 vets before your conversion attempts. That results in a HUGE difference in probability of conversion.

And, last but not least: do you have premium?

Need a scatter chart to show the relations. X will be the vet, Y will be level but I have little idea of how to create such chart.

Also seems having a large number of experts help convert success rate, see entry 135-140.
Edit: entry 160, 162, 167 and 168 shows otherwise. 

One more question, will the success rate be lowered if the vet portion is large in max crew size?
If it does, how to balance it with the lowered success rate due to higher level? (if higher level does lower success rate)

Example - A Level 100 sailor with 100 vets and maximum experts (~500).

Question:

If you burn through all your experts (approx 500), how many vets do you need to acquire in order to achieve the same ability you had with max experts had you done nothing?

Answer:
1 vet is worth approximately 4 experts.

You would need 125 additional vets to achieve the same level.  

Now, if you have a spare 1,000 or so experts waiting to be placed and you can add twenty or thirty vets by your actions and then replace the lost experts with 470 or 480 experts then go for it.  After my sailors reach L100, I pretty much give up wasting experts trying to get more vets because the success ratio is about 0.00001%.

It seems that during the vet conversion events, sailors with 0 vets can convert up to 40 vets
with a 100% rate.    After 40, the success rate starts to fall off.    The 100 percent success rate seems to be in place even if the sailor has 30 vets.    40 vets seems to a breakover in the success rate.

I've never kept an exact statistic but I have done the 0 to 40 conversion enough times to know that the success rate is nearly 100%.   

The lvl of the sailor doesn't seem to make any difference, just the fact that the sailor starts with less then 40 vets.

So, I have never added vets from drops or otherwise until I've rolled 40 vets onto the sailor.

I also had post this one.

http://fm.en.kupaisky.com/Community/Forum/View.aspx?num=20318&searchtype=1&pagecount=0&searchvalue=richardphat&sort=5&category=C01&page1=1 


I have yet to update the new chart.  

On the near 100% chance of rolling 40 vets is the sailor has none,  I've also noted
the following.

Usually, the first 1 or 2 rolls seem to be sucessful.   If there is a failure, it always seems to be the 3rd or 4th roll.    Might be evidence for a sliding success scale.

A very old thread on rolling for vets claimed the best time to roll was after a successful battle (win), and if a sailor or two lvld up.    Don't know if it's valid or not but I've tried it a few times.   Can't say if it seemed to make a difference but it doesn't hurt to do it.

I have noted that, for me anyway, rolling vets for IJN airlift troops to be used in Occupation or HA tends to have more failures.    I can't believe that the programmers would spend the time to make certain sailors harder to roll for vets so this just might be a matter of bad luck.
My success rate for this sailor from 40 to 50 vets during this last event was 50%.   First roll was sucess, 2nd roll failure.    And this was with the increased probability for getting vets during the event.