How do you mount your pony bottle ??

[spectrum]

Sling it.
You can then see it and turn it on and off easy ( you will need to do this on every dive to pressurise the hoses) and see the SPG and see if it is leaking.

Although I do agree that its not really big enough for a deep dive it is better than nothing. (just).

Lets say ( and im working in imperial so forgive any errors) you dived to 130ft (40meters) and you are stressed so your SAC is elevated perhaps 1cf (28litres) per minute. At 130 ft you will consume 5 cf per minute. Its not going to last long is it.
Did the rep at the dive shop tell you that?

I doubt you will want to change your weighting at all.

Let's do the math such as in this oldie.

Pete