Lens too far back in the dome?

So, I admit, despite a frenzied researching session, i am left feeling like a professional double check is needed. 

I am obviously new to the profession, and as such, starting out with the basic equipment. I know Meikon/SeaFrogs is regarded pretty lowly, but its what i can afford, and it seems to be reasonable for my use. That said, i want the best i can afford, so after much reading, wound up purchasing an 8" dome. I currently use a Fuji 10-24mm, so the flat port just wasn't cutting it, especially in the corners. I did the reading, and found that the 78mm extension was recommended for this particular lens. 

I received it yesterday, and am feeling torn. Obviously, this thing is a monster, but i can work on managing that. But i see that the lens itself is recessed into the dome; i feel like i see most dome shots with the lens extending INTO the dome. Upon doing my unscientific test (aka, a pool), i feel like the camera focuses well, and even the subject off to the sides is generally sharp and in focus. I have done my best to read up on the lens theory, and i know how its a virtual image, and focusing happens much closer than on air, but i have no way to calculate the optimal point for this setup. 

Sadly, the only other tube available for this setup would be a 40mm, and i feel like that would put the lens too far into the dome (but maybe i am wrong).  Any thoughts?

What does the port chart say - which brand of dome?  The main impact of being too far back in the dome would be vignetting - if it doesn't vignette , it's close to the right position by the look.   It looks like a 180° hemisphere dome so the centre of curvature is close to the dome base and you can see the entrance pupil which looks like it's very close to the front of the lens. 

at the 10mm end particularly, you will still need to stop down to at least f11-13 to get sharp corners.

Sorry, I thought I had put it into more detail.. the housing is a Meikon X-T3 housing, with 8" dome and 78mm tube. As far as I can tell Meikon doesn't have a specific lens chart published (at least not one I can find for Fuji), but this was the setup that Aditech recommended on their page. 

There is only vignetting at the extreme wide end (10mm) and would only require minor cropping. Past 11mm it seems good, and testing it there in the pool seemed to give adequate results (not that a pool comes close to anything I am diving in). 

Meikon housings/domes are cheap for a reason, they sell something that allows you to get the camera UW and take photos, however, particularly with wide angle they always don't offer an optimised solution,  they work , allow you to control the camera underwater and take a reasonable shot, but often they are not offering something that has the port in exactly the best position.

The minor vignetting you report is probably not surprising and it's probably as good as you can expect.  Looking at it from the photo it looks to be close to the right position, but without detailed measurements it's hard to tell.  All I can suggest is taking it on a dive and trying it out.  If the corners are softer than you like, you could try zooming in a little to close to 12mm.  As you zoom in the corner softness will improve progressively.  This is a common issue with any rectilinear lens this wide - APS-C cameras are a bit easier but still need to be setup properly to give their best performance.  Changing to a 40mm extension would fix the vignetting but I would expect the performance to deteriorate - I assume it doesn't change length as you zoom?

If you get  UW and  take some test shots and if the corners are an issue for you -  you could come back and post some images and their settings we might be able to offer some suggestions from there.  Pool shots in fact allow you to take a decent test shot to check for corner sharpness just get parallel to a wall and take a photo of the tiles at various apertures - they'll allow you to see how sharpness changes with aperture for one thing.  If they don't have tiles take a couple of toys that sink to place in the corners or even a steel ruler or similar.

Appreciate the suggestion for additional pool testing. If the weather hold up ill take it for another dip, as i need to test out my DIY zoom gear as well; so ill look for anything i can take with me to test it out. 

At first glance, the corners are definitely sharper than the flat port, which obviously smear badly at the corners and really only gives a sharp picture in 50% of the frame. I have made it work surprisingly well, but i could see how back the port was holding it back as well. 

IN regards to length - the front element moves a few mm back and forth depending on zoom, but it is minimal; the lens overall does NOT change overall length. 

If you try it out take shots between f8 and f16 to see how the corners vary.   The flat port would be quite ordinary in the corners and would only approach acceptable at maximum zoom, any dome will be an improvement over that.