You usually don't need more than 100 vets and max experts since gunners usually cap out anyway.

IJN AA gunners are a bit different.

Personally i classed mine on time and they work well BVE with 155 vets.
In your case since you late classed them you won't need as much as i have.

I'd say you will be fine with only 109-110 and max experts.

First of all, class them right now. After 62 level armant sailor gain less true ability per level than full classed DP gunner. If they are based +12 i think you don't need more than 100 vet. But at your place i would take profit from current event and veted them to about 150 vets. You will hit reloadcap faster.

Ignore about everything the ppl above have said.

Converting at 90 is indeed a good point to make them DP gunners.
However the moment you class them, they will most likely not have reached the reload cap yet, which is the reason for the delay in classing them. You'll need more levels and/or more vets if you want to reach it sooner.
As a standard 100/109 vets and a good amount of experts is recommended for gunners, but if you feel lucky I wouldn't hesitate to go (much) further. Reaching that cap earlier is always nice, and it might even allow you to shave of some recruits from the gunners when you go over the reload cap ( not exactly known for ijn 4.7", but I guess it's between 2.5-2.7m TA)

Especially with this weeks veteran conversion event going on I would try to convert every expert you have. 300% vet conversion is as high an event as $DE goes.

Since you plan on using AA ( obviously), regaining the experts shouldn't be too much of a problem.

As for gunners not "needing"high amounts of vets on gunners, it's both correct and false.
It's correct in that most gunners reach their caps rather easily ( except DP gunners!!!). so you don't need high vet counts to reach it. But it is false in that with more veterans and some experts you can make gunners considerably lighter. Also for a lot of gunners it doesn't hurt them much to try for more veterans, their ability score and number of veterans will allow them to reach the caps still, despite having almost no experts. When you're shooting for high vets on gunners, do some calculations on how many experts you should leave to still keep them reload capped ( acc cap is lower and will be reached automatically then).
Unless you're loaded don't buy experts to try for high vet gunners though.

A cap is a something that limits the ability of a sailor.

Let's simplify it.
Let's say the cap for accuracy is 1000 units and you have 2 sailors.

One has a growth of 100 units per level while the other has 90.

The first one will reach the cap at level 10 and will STOP improving and they will be identical performance wise when they both hit lvl 11 (co that the other sailor caps out).

This is a bit more complicated than that (the cap is mobile and increases each level a bit) but you get the idea.

Soft Defense also has a cap at 900 units ship wide (it's easy enough to reach so restorers are pretty much a waste unless you drive a CV).

Also don't use up all of your experts unless you have little to begin with have expert packs reserved in your item tab or are very good (and lucky) at AA.

The formula for ability (as well as the caps) is in this thread.

http://fm.en.kupaisky.com/Community/Forum/View.aspx?num=9411&searchtype=0&pagecount=0&searchvalue=&sort=5&category=D01&page1=1 

As for using up all of your experts... don't.
10 vets are worth about 40-42 experts but you really need a way to replenish them via expert packs or AA.

Every sailor is actually a groups of sailors and has a set number of sailors divided in 3 groups.

Rookies.
Experts.
Veterans.

Each have different value in counting true ability.

Rookies are worth nothing (but are still needed because not having full crews inflicts severe penalties)
Experts are worth 1 point and veterans are worth 4.

"Since SDE recently introduced that ability percentage of every sailor is shown you can

also calculate the true ability by 

(skill percentage * displayed ability * max crew),"

Let's take a random sailor from google.
 
His total skill percentage coming grom vets and experts = 90% 
The displayed torpedo ability is 1489
And the max number of sailors is 489 

So to calculate his true ability we have to  multiply them.

90 x 1489 x 489 =  65530890
Aldabert's guide states that torp reload caps out at 850k ability  meaning that this torper  is way above the cap since torps reload 50% faster at 850k ability and reload stops improving after that.
So in this case the sailor is either several times above the cap or i derped the calculations somehow and ended up with extra digts.